Trigger Warning: Sensitive topics below.

In a post I made last year, I pointed out the small skull in this display is case belongs to a baby.

The 'crack' down the center of the forehead is a frontal suture which closes between the ages of three to nine months, and to showcase the fact it's not just a cracked bone, the animators included a fontanelle between the bony plates of the skull, which closes before the age of two.

There is the skull of an infant in the Belmont Hold.

So what's the story here?

Theory 1: The Skull is a Vampire

This could have been a human baby that was turned by a vampire for whatever reason. Maybe this was some vampire's twisted idea of a pet. Maybe it was a horrifying, psychological trauma where a human woman lost her baby and after becoming a vampire, turned an infant into a vampire so as to never experience that pain again whilst tragically blind to the fact an immortal infant would never grow up.

If either of these were the case, whichever Belmont found this child could have viewed his or her death as a mercy. Since the baby would be immortal, it would be impractical to keep it alive forever, not to mention almost cruel because what kind of life is that? And it would be equally inhumane to leave the vampire to starve to death, so the options are limited.

The skull was brought home not as a trophy but as a specimen to study and serve as a grim reminder of a harsh reality.

...

Theory 2: The Skull is a Dhampir

Carmilla's above comment gives me the vague impression that it's not unheard of for a vampire to have a child with a human and then turn the human into a vampire, creating a family unit. (Just the way she says it. Like the unspoken line was, "You married a human, you succeeded in getting a child off her, so she was free to become a vampire after, no?" Turning Lisa after Adrian was born was the next natural step in her mind, so it baffles Carmilla why Dracula's wife remained human.) So maybe dhampirs are uncommon, but Alucard is far from the first one to ever exist.

So if the skull belonged to a dhampir like Alucard, this opens up another set of possibilities, but before I go into those, I want to address the Belmont Clan's potential view towards vampires and human-vampire hybrids. It's not clear if Alucard needs human blood to survive. He eats human food, but so do the vampires. Lenore comments they get their essential nutrients from blood, but whether or not this also pertains to dhampirs is up for debate. In the Gresit Underground Keep scene, it does look like he had some form of blood transfusion system possibly sustaining him, but this could arguably be a life and death situation. As in, he doesn't need blood to survive, but it can also save his life if need be.

There's also the possibility the blood-drinking is not the same across the board. (Say Alucard doesn't need blood to survive, but another dhampir was saddled with the shitty genetics that make blood a requirement.)

For the Belmonts, I can see dhampirs being a controversial subject. From Leon to Trevor's time, there are four centuries and countless individuals with their own set of similar but unique values and opinions, so it would be understandable for various Belmonts to have points of contention as well as shifts in viewpoint down the generations as new information about the enemy comes to light. Some members might have the stance of, "Dhampirs do not need blood to survive, are not a threat to humanity, and therefore it is unnecessary to hunt and kill them," while others may have taken the more extreme stance of, "Dhampirs are unnatural creatures that do not belong in this world any more than vampires do." Without the precedent of Alucard and Trevor having a common enemy, there would be nothing to sway the entire family one way or the other.

So if this is the case, the skull could have come from a vampire family similar to Dracula, Lisa, and Alucard's, and that family could have been discovered by a Belmont who made the decision to eradicate the them, including the dhampir for whichever reason:

The existence of dhampirs hadn't been discovered/confirmed yet, so the infant was assumed to be a vampire and it was viewed as a mercy killing out of ignorance. (Remember, Trevor thought Alucard was a vampire when they first met, so there is no visual difference between the two species.)

Dhampirs are assumed by the Belmonts to need blood to survive and are therefore a predator to humans that needs to be put down, again out of ignorance or just plain malice.

This particular Belmont was a heartless psychopath who had no qualms killing an innocent baby just for existing.

...

Theory 3: It's a Dhampir and the Belmonts Have a Really Fucked Up History With Dhampirs

There is also the possibility the mother of the dhampir child was a human woman that was raped and impregnated by a vampire. (Surviving somehow...?) In this scenario, there is a woman terrified by the idea she is about to birth a monster and goes to the local vampire experts for help. The Belmonts take her in until she delivers this unwanted child and the fate of the baby dhampir is entirely in their hands, which brings us back to the mentioned controversy among the family members.

If the mother abandons her 'monster' child, the Belmonts are left which the choice, "Do we kill it before it grows up to kill someone or do we give it a chance and let it live?"

Another possibility is the dhampir was brought up within the household for the purposes of 'studying' or 'rehabilitating' its nature. Say the family discovers dhampirs don't need to subsist on human blood and are relieved. "Wonderful. Dhampirs can live as humans and there is no need to kill them." The dhampir grows up happily among his or her adopted human family.

But then there is a tragic accident where he or she doesn't know their own strength and fatally harms a family member. The Belmonts then make the decision: Lock them up or execute them as a monster.

Based on this precedent, the family then closes off the idea that dhampirs can live as normal humans forever and they subsequently commit infanticide against any future dhampir children that cross their path, a blanket decision based entirely on a one-time misfortune.

Theory 4: The Belmonts Were Not the Cause

This again calls into question how dhampir physiology works, but maybe the infant died of an illness of failure to thrive. Alucard is strong and healthy, but that may not be same for other dhampirs, especially ones that may have been rejected as a 'freak' by both humans and vampires. He had the advantage of having a loving family and a safe environment in which to grow up. Other dhampirs might not have had the same good fortune and their health suffered for it. (Or they got dealt a bad hand in the gene pool.)

As for how the skull wound up in the Hold, it could have been one of the 'weird stuff' the Belmonts found and brought back home with them.

This could also apply to a situation where a vampire killed a rival family and kept their skulls as trophies, and the collection simply wound up in the Belmonts' treasury after destroying said vampire.

"It's like a museum dedicated to the extermination of my people, so no. Not thrilled."

Whatever the truth of the skull, I'm inclined to believe Theories 2 or 3 because this scene struck me as a very subtle nod to how no side in a war is completely innocent. On the one hand, vampires like Carmilla, Cho, Godbrand, etc. hunt and toy with humans for sport and view them as lesser beings, and so the Belmonts seeking to eliminate them would be an understandable measure to protect humanity.

On the other hand, the presence of the infant skull indicates a tragic and bloody history of poor choices, old prejudices, potential atrocities committed by the 'heroes,’ and generational trauma. A past history even Trevor isn't particularly proud of for all the pride he has in his lineage. It really showcases the line between man and monster and aligns with the recurrent theme of 'we can be better than this' that occurs throughout the series.