Ponies, inherently, are cute. Animals in costumes, inherently, look existentially tortured, because they cannot talk (except for, perhaps, certain birds), though it's clear if they could, they'd explain how domesticated animals don't enjoy wearing sweaters for any holiday, no matter how "cute" their owners' Instagram feeds say they are via hashtags.
Somehow, though, the cuteness of a Shetland pony in a onesie trumps any of that existential dread. I mean, look at this guy. His name is Daffy. And he looks dapper.