Fluffies, being the chimeric hodgepodges they are, are very unstable genetically. This in turn makes them more prone to mutations than any natural animal, with pre-revamp and badly bred fluffies being extremely prone to it.
While some mutations are more or less benign, others are very detrimental.
One mutation that can be either depending on the circumstances is Faulty Wing Syndrome, a defect which only affects pegasuses, alicorns, and winged fluffy subspecies like batfluffies.
As the name implies, FWS comes from a mutation in the gene that codes the growth of wings, which are one of the most genetically unstable aspects of fluffies. The Hasbio geneticists first working on them had to figure out a way to encode the growth of an additional pair of limbs into a tetrapod-based creature, as well as ensure it would produce predictable and desirable results to make pegasus and alicorn fluffies possible.
FWS occurs when the gene responsible for the growth of wings instead grows legs.
Though benign in itself, the potential harm from FWS comes from the reactions of other fluffies.
Even fluffies without color bias or mental defects like CAD or Smarty Syndrome can react badly to a mutation like this, running away from the “munstah” in fear — which can be dangerous if this reaction happens right after birth as it often does — and some more ill-tempered fluffies will even stomp the “munstah babbeh” to death.
An equivalent mutation is present in pegasus and pegacorn fuzzies, though it seems to be somewhat rarer and is much less studied.