#shipping warning

However, it’s also possible its growth is partly based on skeleton “genes”, for lack of a better word. If the two breeds grow at the same rate, the effects would cancel each other out, and the growth rate would be normal.

Still, if the growth rate of each is different (i.e., if Boss Monsters take much longer to mature than skeletons) it would be hard for either to figure out when to expect developmental milestones. What if Asriel made his first magical bullet at sixteen, but Sans made his at four?

Active Vessel

(Diagram supposing the child doubles the drainage rate and connections with Toriel, while not draining Sans)

This is the most dangerous possibility, not to the child but to the parents. This supposes the flow of SOUL power is something like mammal pregnancy, where a complex chemical interchange keeps the interests of the mother and embryo in balance.

If unable to draw the usual amount of SOUL power from one parent, the hybrid's body/SOUL might double the drainage rate on Toriel, making her age twice as fast. However, if a Boss Monster hybrid drains whoever registers as its parents, regardless of breed, Sans’ “SOUL power” might flow to it as well.

Assuming it’s like human pregnancy, Boss Monsters’ bodies might be adapted to withstand the draining of SOUL power. (whether by controlling drainage rate or just by having a lot of SOUL power to start with) If skeletons don’t reproduce the same way, though, Sans’ body would have no adaptations to the drain of SOUL power. If he is “the weakest enemy” (e.g., has a weak constitution and presumed low SOUL power) and the child’s draining is particularly aggressive, Sans might be rapidly drained and die.

Sans’ body probably couldn’t counteract this. Again assuming pregnancy parallels, his child’s skeleton genes probably couldn’t counteract it either. Ligers are so big because they have a lion for a father but a tiger for a mother. The lion father's genes encourage embryo growth, while tigresses (unlike lionesses) have no means of suppressing embryo growth. Indeed, it’s possible the child might inherit all the aggressive draining genes but no “whoa there, that would kill your parent” suppressing genes.

Why Sans Would be a Good Father

That title was somewhat misleading: there's no evidence Sans would be an especially responsible or attentive father. But he might be great for siring hybrid children. Again assuming soul power draining is like an embryo drawing nutrients from its mother, it might be good that Sans is the father.

In mammals, the father's genes build the embryo's side of the placenta, an organ the embryo uses to get nutrients. The father's genes encourage the placenta to draw in as many nutrients as possible; the mother's genes (in her body) act to weaken it.

If the child itself causes the flow of soul power, and the power-draining trait is influenced by imprinting like the placenta, then a hypothetical Boss Monster/skeleton hybrid with a skeleton father might have weaker soul power draining. Theoretically, Toriel, or the pair overall, wouldn’t experience the same strain as Asgore and a hypothetical female skeleton would.

Where potential mates of their own species are rare, animals might hybridize with closely related species. Furthermore, animals might make hybrids if they're compatible species kept in the same enclosure (e.g., lions and tigers in zoos). In birds when one species parasitically lays its eggs in the nests of another species, the offspring might imprint after its host parents in looks or song, and so only be interested in birds that look or sound like its host parents.

All these things likely happened when monsters were sealed underground. While it’s unclear if different monster breeds interbred before the war, given monsterkind’s massive casualties it’s likely monsters’ options within their own breed were limited. (This would surely be especially true for monster breeds that were rare to begin with.) Staying underground, especially if they all stayed in just one city for a long time, would also work like captive conditions. Furthermore, the devastation might have meant a whole lot of orphans, which might have been raised by different breeds. Such orphans might, like the parasitic birds, prefer breeds like the ones they were raised by. It’s certainly clear monsters have variable preferences and can imitate other monsters (e.g., Papyrus catching a thrown stick in his mouth)/have variable behavior, after all.

Biological Problems

For some breeds, even if hybridizing is possible it would likely cause health problems. This is especially obvious for fire monsters, such as offspring of the Sans X Grillby ("Sansby") pairing. Though skeleton monsters are apparently1 insensitive to cold, it's unclear whether they're insensitive or immune to heat too. Fan works often assume Grillby's fire is painfully hot, and becomes much hotter when angry. While (dry, human) bones can withstand greater temperatures than human flesh without (visible) damage, being on fire constantly might be a problem. Even if the hybrid children can withstand it, it's possible (and more likely) that they might hurt themselves from the higher temperatures if they ever get angry.

Pain can make people easier to anger, so being burned by their own flames might cause a self-destructive feedback loop of pain and burning. If they can't quickly heal burn markings from such events, they might become anxious about it, akin to obvious scars or burn wounds.

Environmental Unsuitability

Environmental unsuitability is also a problem. While a hybrid's parents might be well-suited to distinct environments, the hybrid may be unsuited for either or require unpredictable intermediate environments.

For example, the fish monster Undyne is vulnerable to heat stroke/dehydration when travelling through the dry, hot caverns of Hotland; Alphys is not. Evidently travelling through Hotland is enough of a danger to Undyne that Alphys installed a water cooler for her halfway to her lab, even though Alphys' lab is only a few screens into Hotland. While Undyne says she dislikes "snow", she's evidently not in danger standing outside Papyrus's house indefinitely. This might not be true for Alphys: while some fish can live in cold water, reptiles either die or hibernate in cold environments like Snowdin.

A hybrid child of the two might do well in an intermediate environment (e.g., hot but humid, like south Florida) or take after one or the other unpredictably.

Some assume Undyne can breathe underwater through gills; what if the child prefers most of their body to be dry but needs to keep their gills moist? What if the child prefers dry, but not hot, environments? With such a potential blend of preferred environments, they all might need custom rooms (e.g., humidifiers, pools) in one house.

Most dangerously, what if Undyne can breathe water through her skin, but a reptile-skinned but otherwise fishlike child cannot? One work, Scramble Saga, suggests Undyne sleeps in a water-filled bathtub rather than a bed. If a helpless baby that can't breathe underwater is put in a tub-crib, the baby might drown. The child might drown, too, if Undyne is playing boldly and aggressively in the water and mistakenly assumes the child can breathe underwater.

Psychological Problems

No Horns

(The picture is based on goat skulls; notice they don't have horns. Picture modified from here.)

In goats, horn buds don’t grow directly from the skull, but from a specialized patch of skin. Only later do they start to merge to the skull.

Assuming Boss Monsters’ horns grow like that of goats, a Boss Monster-skeleton hybrid might not have horns. (Many such fanchildren have no skin, after all.) While many monsters do not have horns, this might still make the fanchild self-conscious about not having horns. (Perhaps similarly to Kris of Deltarune, though Kris is a human.)

The Problems of Papyrus x Mettaton

“Us ghosts spend our whole lives looking for a proper vessel. Slowly, slowly, we grow closer to our new bodies... Until one day, we too may become corporeal beings. Beings able to laugh, love, and dance like any other...” - Mad Dummy

It is unclear whether ghosts’ desire for vessels (or “bodies”) is biologically or culturally driven. Either way, hypothetical fanchildren of Papyrus and Mettaton are likely to be dissatisfied with their bodies.

LordLaharl’s Vassal made a convincing point that Papyrus may have body dysmorphia issues. The Wikipedia article on the subject suggests part of the variation in body dysmorphia issues in the population (or “heritability”) can be explained by genetics. If this is true, and also true for monsters, a Papyrus x Mettaton child could have twice the body issues of either parent.

It’s unclear whether the child would be born/the equivalent of born a ghost fused with a skeleton body, a ghost possessing but not fused with a skeleton body, or be born separately as a ghost body and a skeleton body. If the Papyton child has a handy skeleton body from birth/the equivalent of birth, the child might not develop ghostly psychological problems, as the child could do normal non-ghost things with it.

Related Reading

Inter-Species Socialization

Do Monsters Have Instincts?

Chapter 1

His back pressed against the ceiling, thankful that he worked nights as he watched the police from the shadows. Floom... The coppers shone their lights around the gallery and their voices floated up to him, all of them familiar by now- the cloying, high-pitched squeals of Officer Bashful, the nasally laugh of Detective Sneezy, and the deep merry laugh of Detective Happy. Good, he's not here yet... He slowly edged his way toward the open window he used to get in, holding his bag with the necklace inside of it. Very slowly... Almost there... Detective Sneezy let out a huge sneeze, which covered the small nose he made as his foot scraped the windowsill on his way out.

Sleepy heaved a sigh of relief as he stood on the balcony. "That was close..." he whispered.

"Oh, don't celebrate too soon, Sleepy."

Floom.

"Well, gee willikers, it sure is nice t' see you. How ya doin', Doc?" he asked with a cheeky grin.

The Chief of Police stood with his one hand on his gun, the other holding his handcuffs. He took a step forward, "Let's not put up a fuss this time, okay? I'd hate to have to call in the coroner at such a late hour."

Sleepy took a step back, "Oh, I agree. It wouldn't be nice."

"No. No it wouldn't."

They glared at each other with fake smiles, Sleepy slowly edging backward as Doc advanced. Without warning Sleepy turned and bolted, vaulting over the rail. A bullet whizzed by his head, followed by a curse from Doc. On his way down, Sleepy stuck his arm out to catch the gutter, whistling as he slid down the pipe. Another two shots fired, both hitting the gutter- Doc was off his game today! He heard other whistles blowing as he hit the ground, police whistles; he kept his ears open for--

There. That was it.

A black truck driving on the wrong side of the road pulled up alongside the building, the driver sticking his head out the window and whistling. "Heya, Dopey!" Sleepy called casually, as though he were going for a jog, running around the front to jump in the passenger seat. "Grumpy's here, too, right?"

Dopey nodded, jerking a thumb towards the back as he floored it. There was a yell from the back of the truck, "Goddamnit, Dopey, warn me before you take off like that!"

Sleepy turned toward the back and smiled, "Hey, Grumps, Doc says hi."

"Oh, he does, huh? Lemme say hi back." Grumpy loaded the tommy gun he held, rolled down the window, and opened fire at the police cars now in pursuit. "Sleepy, you better have a good explanation for this shit! How the hell didja get caught?"

"Alarm went off."

"You were supposed t' disable it!!"

"No, YOU were supposed to disable it, asshole! This is your fault!"

Dopey jerked the wheel just enough to rattle them and get them to stop arguing. Properly chastised, the friends continued on their way. "So how're we supposed to lose them? I don't exactly have all the ammo in the world, y'know!" Grumpy yelled.

"Where're the Glooms at? You told 'em it was tonight, right?"

"I THOUGHT YOU WERE GONNA TELL 'EM!!"

"GODDAMNIT, GRUMPY, YOU HAD ONE JOB!"

Dopey sighed and rolled his eyes, turning the wheel sharply down another street- he knew a few places to hide, tricks to help lose the heat. The wail of the sirens behind them echoed throughout the dark Jollywood night as the truck sped along, finally exiting the town.

"Dopey, where the hell are ya goin?" Grumpy shouted.

"Shh, don't bug 'im, he's probably got a plan!" Dopey gave a thumbs-up as an affirmative before driving into a corn field. His two friends screamed in panic as they were suddenly blinded by row upon row of corn hitting the windscreen. The driver ignored them, and turned the wheel again once, twice, now headed back the way they came. He stopped right before the end of the field and turned off the headlights.

Grumpy started shouting again, "Dopey, what the f--"

"Shhhh..." Dopey calmly placed a finger to his lips, then pointed to his ear, indicating they ought to listen. 

So they listened. 

They listened to the sirens as the police followed their path into the cornfield. And they listened... And listened... And then they heard a loud -bang!- followed by several other crashes.

"Hey... Isn't this that old woman's field with the ditch that runs right down the middle of it?" Sleepy asked finally.

Dopey nodded, a wicked grin on his face.

In the distance, they could hear a man swearing loudly. "Ooooooh, Doc's pretty pissed!" Grumpy snickered.

Sleepy laughed and pulled out the necklace; a flawless string of diamonds shone faintly in the dim half-moon's light. "When we pawn this off, remind me t' buy him an apology card and some flowers."

***

Humidor adjusted his magnifying glass to get a better look. "Yes... Yes, this is fantastic! I'll give you boys one hundred dollars!"

"What?? Hell no, that's worth at least two hundred!"

"Oh, to be sure, Grumpy, but you don't understand how this business works. I need to find a buyer that won't ask questions, and even then I may not get full price for it, you see. If I want to make any kind of profit I have to give you one hundred dollars."

Grumpy balled up a fist, ready to say more, but Sleepy stopped him with, "Hey, take Dopey for a walk." The angrier man glared at the both of them before stomping outside with Dopey trailing behind. "Sorry 'bout that. Is that really the best you can do?" 

The older man shrugged. "I'm afraid so."

"Eh, we'll take it." Money changed hands, and Sleepy walked out into the cold November morning, yawning expansively. Thankfully the truck was warming up, he needed to catch a nap on the way home.

"So what'd he say?" Grumpy asked as he clambered in.

"One hundred dollars," Sleepy shrugged. Dopey put the car in drive and sped off.

"Oh, great, that's perfect," Grumpy spat acidly. "I hope you got ideas about how t' get cash quick, 'cause we're three months behind in rent! At this rate, we might as well just live in the truck. Which, by the way, is almost outta gas! And is now completely recognizable by the cops, may I add, and I'm almost outta bullets so we can't keep them off our asses for long!" Sleepy snored in his seat, his head drooping forward. "Unbelievable." 

Dopey grabbed a newspaper sitting on the dashboard and handed it to Grumpy, who flipped through the pages until he found the want ads. The truck bounced along for a few blocks, then Sleepy's head popped up. "STOP."

Dopey hit the brakes, sending Grumpy flying half a foot forward. "What the--" Sleepy jumped out of the truck and ran into a flower shop. "Oh my God, he was being serious," Grumpy snickered.

Dopey pulled out a pencil and drew a heart on the newspaper.

Grumpy blinked. "No... you don't think?"

His silent companion nodded, then pointed out the window. The sight was enough to make Grumpy's jaw drop.

***

The chief entered the station without a word and made a beeline for his office. Bernard piped up, following his superior as he charged ahead, "Uh, Chief, there was something delivered to your office, I-I signed for it--" he was interrupted by a door slamming in his face. He sighed forlornly.

"Heh, you're tryin' too hard, Bernie," Harry called from his desk. "'Sides, right now's prob'ly noooooot the best time, the DA called him in."

Bernard bristled at being called that. "Starchback? What'd he want?"

Simon turned in his chair. "Dunno, but every time Chief gets a back from Starchy's office, he's in a foul mood for the rest of the day." He winked at the new officer, "It's okay, Bernie-"

Bernard said very quietly, "Don't- Don't call me Bernie..."

"-you'll catch on once you've been here a few more months. Oh, and you might want to move away from the door riiiiiight..." He looked at his watch. "NOW."

The young man stepped to the side as the door flew open and Chief Farmer, his face red with anger, threw a vase full of red roses on the ground, shattering it instantly. He bellowed, "RICHARDSON! MENVILLE!"

"Yes, Chief?" The two detectives didn't even stand.

"Any new leads?"

"No, Chief."

"Goddamn it get on it! I want that sonovabitch found yesterday!"

"Yes, Chief."

Chief Farmer turned and hollered, "WEST!"

Bernard blanched visibly and tried to control the shaking in his knees as he answered, "Y-Y-Yes, Chief?"

"You signed for this??" The enraged chief pointed to the shattered vase.

"Y-Y-Yes sir..."

"Who delivered it??"

Bernard stuttered nervously, "Ah- he ah- i-i-it was-"

"SPIT IT OUT, DAMN IT!"

"It was th-the regular delivery-man, sir!"

Chief Farmer's eyes narrowed, his mouth set in a firm line. "If that delivery man ever changes and I'm not there, you hold him in custody until I get back, get me?"

"Y-Yes sir! Ah..." His curiosity got the better of him, unfortunately, "who were the roses from?"