Simply put, they are mentally handicapped.
Unlike a “Derp”, which is one either physically or emotionally (because fragile cartoon horse in flesh can die from sad or become mentally broken from excitement) braindamaged, a Sensitive is one that was that way from birth. Previously all neurodivergent Fluffies were Derps so this term has replaced Derp for birth defects.
They’re fairly new idea that was only expanded on very recently, and in different ways by different people, so you have an even bigger blank check than normal.
The basic concept is they are developmentally stunted. Though different things in the same canon from autism to severe debilitating dwarfism may present, Fluffy programming and biology gives common results though some writers use it as a unique condition to Fluffies. As you may imagine the writing uses it as a stand-in for real issues, allowing the author and audience to indulge in either the sadism if brutally murdering or torturing a mentally handicapped person, or as an even more vulnerable Fluffy to protect.
The term is what other Fluffies call them, and thus what humans call them. “Mummah’s sensitibe babbeh.” The term is kind of meant to make you hate them by triggering that insecure rage, though it may have the opposite effect for you.
Some past versions:
-
They are rejected at birth by their mothers like any Derp, and must be kept alive by human care. Possibly abandoned to die of exposure/starvation, mercy killed with a stomp, or eaten. There is no reason for them to survive at all aside from setting them up for torture, or as a cute little wooby darling. They’re basically a Chirpie with closed or barely open eyes. Unlikely to live long even with care. They basically just have no reason to exist, and will not for long even with the best care, though giving them a happy if short life makes good Hugbox, while for Abuse its basically just a Chirpie who will die horribly even if they do nothing.
-
They are permanently a baby, requiring lifelong care which their mother will provide either out of genuine love or the baby smell chemical depending on how much free will the writer assigns them. They may have some degree of survival outside her care via a mounted bottle in their later life, and may indeed survive better due to their inability to ever speak or wander off. They sometimes can toddle about somewhat, but ultimately their severely stunted mobility means even a Splorin will not leave the play area. They will understand they need to defecate and attempt to reach a litter box but likely fail, needing to peep to inform their mother or caregiver they need to be delivered to the litter box. The mother who cares for them will be unlikely to have a Bestest or reject colors/Alicorns, but the Sensitive may cause a resource drain and cause the deaths of the siblings. I don’t recall a story where a mother of one has to choose between her young yet. They are superior as a pet to #1, and may be endearing for their chubby useless bodies and permanent baby brains, or possibly as a way to provide a baby-crazy Fluffy female with one for life. By the opposite token they can be a fake-out from a can or breeder, getting something seemingly valuable but actually useless. These are the best use for abuse because they can actually react, having a high degree of awareness while the simple mind and severe vulnerability triggers extreme cute aggression.
-
Mentally handicapped in human ways. Social issues, sensitivity to stimuli, learning conditions like a normal Fluffy if from a breed that is more intelligent since otherwise you could never tell the difference, the need to stim themselves with texture or posture, extreme obsessions with things, collecting things, and so on. More for concept exploration, since Abuse makes it clear the Fluffy is a stand-in for beating a handicapped human. Presumably they come in a vast range of issues, some capable of learning to live like an adult or passing as normal, though I don’t recall seeing this explored anywhere (a Fluffy telling decent fanfiction or making fairly good artwork to the amazement of regular Fluffies, maybe finding a specialist place in a Herd or as a facility role like Nurse Fluffies but as an entertainer seems a bit on the nose).
There is probably more concepts, but its what I recall.
If you want the origin story, its an Abuse one here.
It got a comic adaptation as well.
Additional art by a different artist.
https://fluffy-community.com/t/illustration-of-no-miwkies-author-foalout4-art-by-fiddwe-46609-46625/5328
My general notes: if Abuse it is a way to amplify the aggression by increasing vulnerability and cuteness while triggering a hatred response to its status as a “mooch” but always involves using its inability to care for itself or explain what is going on even in Fluffy terms to prolong its misery, then punishing its siblings for allowing it to live and finally its mother for producing it or for showing it mercy, though unusually avoiding more common misogynistic tropes.
If Hugbox the story is mostly about the human and the Fluffy. The mother is barely a factor.
I don’t recall seeing Neutral, though I assume it would be more a case study and analysis of how it would survive, possibly thrive in some way. Making it functional in later age and with autist artist powers could factor into a story analyzing Fluffy culture and regional differences, as only established and successful clans could support them.
But its mostly an Abuse subject.