several months ago prior to the proper release of the Happy Ghast (soon after it's announcement) I made a short post theorizing about Ghasts.
So, I really like Ghasts from an outside perspective. They're really interesting mobs and they've got a lot of lore going on especially with this new update. That being said I still fucking DESPISE their existences with every fiber of my being going into the nether and being victimized by the quantum ghast (bedrock edition) is infuriating.
To start at a baseline here is what we know CONCRETELY about Ghasts (Only stuff DIRECTLY in the game, no speculation yet)
[ESTABLISHING EVERYTHING WE KNOW FOR CERTAIN]
1. Ghasts have regenerative properties. Ghast Tears are used for regen potions & Happy Ghasts are able to passive regenerative overtime without the need for food (Which is counter to how almost every other regular mob needs food to heal).
2. Ghasts have a full life cycle but we only see Adult Ghasts in the nether. (Because the Dried Ghast turns into the Ghastling then the Adult Ghast / Happy Ghast)
3. Ghasts are capable of flight because of their body structure, not because of the nether itself*. This is because the Happy Ghast was made for over world usage so can be used as direct confirmation of their flight being biological instead of the result of a nether properly (Such as riding air currents.)
*I would otherwise be inclined to stay neutral on such an idea. Because Ghasts are nether creates and taking them out of the nether does not actually tell us if them still being able to fly is lore intention or just a hold over of them being in the nether and not having special coding for the overworld. But the Happy Ghast proves it's a biological function.
4. Dried Ghasts seem to require soul sand to be created.
5. Dried Ghasts ARE NOT rare, as such Ghast Reproduction (However it is done) occurs commonly. Piglins are willing to give the player Dried Ghasts at a roughly 2% rate (So they're uncommon but not exceptionally valuable to the Piglins like magical items such as Potions and Enchants.)
They also have a fairly high spawn rate of 1/3 at nether fossils. Given how common these fossils are in the soul sand valley you're likely to find several Dried Ghasts in any given biome.
6. They drop a music disc, sometimes. Okay so, Dropping the purely scholarly tone for a moment. I do not understand music discs as drops for mobs. Why do Creepers drop them ONLY when killed by a skeleton lore wise? Idk! Why do Ghasts now drop a music disc when killed by a fireball? Idk! Ghasts seem to be biological (or biomagical) entities, not constructs. So them having discs inside them does not make sense. As such I really just have to scratch music discs off as an easter egg thing and not a directly important lore thing. If you disagree feel free to proper an alternative.
(I do love the track though Tears is a really good music disc)
7. Happy Ghasts and Ghasts have drop variances which means the regular Adult Ghasts' drops are a result of living in the nether. Ghasts drop Gunpowder, but Happy Ghasts do not. Ghasts drop Ghast Tears, Happy Ghasts do not.
[SPECULATION]
So what can be established from this information? Well first we can establish that the Ghast in the Nether drops tears because it's a survival mechanism. The Ghast has noticeably more pronounced facial stains than the Happy Ghast. Due to the name ghast 'Tear' and the facial markings beneath their eyes resembling Tear stains, I believe it's safe to assume their tears come from their eyes like other animals. Tears have regenerative properties. Notice how the Happy Ghast still has tear stains? This actually does make sense given it's regenerative properties. We can assume a Ghast is able to heal itself naturally using the fluid that turns into Ghast Tears while it's inside it's body, but it can speed up it's healing by crying; This is why Nether Ghasts have so much more pronounced tear stains than Happy Ghasts (who do not actively need to cry to heal themselves as often).
But I also think the tears serve a secondary purpose. They're also reproductive fluid. It's a VERY well established Minecraft lore tidbit that Gender as we have it in the real world just does not exist in Minecraft. Any two creatures of the same species are able to reproduce (given they reproduce in sexual pairs) but Ghasts aren't able to. You can not breed Happy Ghasts together to create a Ghastling. This means Ghasts do not reproduce Sexually AT ALL. As previously established you can craft a Ghastling.
Since Dried Ghasts are always found in soul sand valley biomes it's more than safe to assume Ghast reproduction is tied to this mechanic. I believe the ghast reproductive cycle goes as such;
1. Adult Ghast goes to a Soul Sand Valley. It cries and releases tears into the sand beneath.
2. The sand soaks up the tears and the tears use the energy of soul sand to grow. (Because soul power is a well established energy source in minecraft due to the skulk, enchanting, exp, soul sand, etc)*
3. After consuming enough energy from soul sand a Ghastling (or perhaps the stage of life prior to the dried ghast) comes out of the sand.
4. The Nether Ghastlings are then subject to an immense amount of competition both from other creatures in the nether and their fellow Ghastlings.
5. The Ghastlings that survive early life turn into full Adult Ghasts and the life cycle continues.
*This process is forcefully stimulated by the Player adding ALOT of tears to soul sand, which is how Ghasts can be craftable and still biological.
Note how Ghasts are not hostile towards each other in game. I believe this is because once a Ghast has reached adulthood it no longer needs to see other Ghasts are direct threats to it's survival.
Another thing to note is that Happy Ghasts do not drop Gunpowder. I believe this is due to their environment. Ghasts use their fireballs, for some reason? It's not very clear aside from attacking the player. Maybe they use it for self defense, or to fight other Ghasts? Hard to say. But I think they're able to create their fireballs due to their diet. Ghasts don't show many signs of being carnivorous. Happy Ghasts feed on snowballs and require water to become Ghastlings. I don't think the snowballs are because of a temperature requirement; I think it's because Snowballs are the most easily accessible form of water to feed to a Happy Ghast. The other options would be glass bottles (which aren't really made for non-humanoids to drink from.)
So I think Nether Ghasts' diets consist primarily of nether fungus, which is where they get their water from. Since fungus is the primary source of food in the nether it's also safe to assume it's the primary source of water. I could also see Ghasts eating loose rock deposits such as gravel or netherrack (If you subscribe to the theory that netherrack is actually a fungus infected stone block) as a basis for the explosive materials in their fireballs.
Now for the main question;
Did the ghasts come from the overworld or the nether?
Well to put simply, They're from the nether;
Just the nether from before it's modern collapse state. I'm not a subscriber of frozen nether theory (I think it's more likely it was something more temperate, perhaps something like a humid jungle with fungus taking the place of regular plants). I think Ghasts are original inhabitants of the nether which fed on the natural foliage of the pre-apocalypse nether (Soul Sand probably? still existed at this point in time). Due to their regenerative properties they were able to survive the nether's mass warming.
I imagine the pre-apoc nether would look reminiscent of the lush caves biome. The Lush Cave has similar structural conditions to the nether, being underground, capable of sustaining life, and having it's own semi-enclosed eco system. But the nether would obviously be a far more extreme form of this. There was probably Lava in the nether beforehand and with the modern nether we know fungus isn't a very flammable block, which is why I lean towards it being a fungus based jungle-ish area instead of a plant based one.
So for my interpretation just imagine a lush cave with more lava, the moss is fungus, and instead of regular stones and such there's stuff like black stone and basalt* instead.
*I'm not totally sure if Basalt would be there since it requires very cold temperatures to form in the minecraft universe, assumably this pre-apoc more tropical nether would be able to provide that in certain areas.
If I had to give them a category animal wise I'm inclined to lean towards some sort of fish. I've heard Jellyfish quite a lot and I think that's a very valid interpretation. But they seem far too cognizant to be Jellyfish due to how sedentary Jellyfish tend to be. I also don't think they're siphonophores (But it's technically possible due to their regenerative properties and their tears acting as reproductive fluid (being smaller ghast fragments in this theory??)). So I'm still undecided beyond them being some sort of classification of fish.