#after getting possessed and being used as only a host body for the ghostly equivalent of a parasite?

[argentdandelion]

Possible Health Problems of Hybrid Fanchildren

No, think of the children!...your hybrid children, that is. Contains lovely ideas for angst.

Hybrid fanchildren are common in the Undertale fandom. It is likely this is because the main characters of the game cover multiple "breeds" of monster, and there are in fact canon cross-breed couples (Undyne and Alphys, RG01 and RG02). Fanchildren are also a creative exercise where one combines the looks and personalities of the parents into a new, distinct character design.

In-universe, if hybridization is even possible, it's likely common. However, hybridizing could cause problems.

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?

Dialogue from Papyrus is contradictory; he explains how he can live in Snowdin's cold environment saying "I have no skin" but complains about the Annoying Dog having cold feet. ↩︎