To better explain the pun, cryptids(chupacabra, loch ness monster, big foot and the like) are refered to in japanese using the acronym Unidentified Mysterious Animal, UMA. However, the japanese word for horse, as memtioned in the translation notes, is uma. So the terms are homophones, and there's the joke.
Wait, how do we know this is Tupai (Remi's pet) and not some random chupacabra?
ChupaAs in chupacabra, I presumeDo you have any different ones?Ta-da, an uma costume.Uma? Mm, I'm pretty sure I do, if you're fine with that.Kyaa!Wanna try it on?UMAUnidentified Mysterious AnimalWaah! She's so warm!Reimu!Hmm...And soft, and fluffy, and snuggly! ❤Like a uma costume from last year?
As in uma, the Japanese term for horse. The joke may be easier to understand if left in romaaji like this.I only have one sheep costume, though.I wanna wear something like this, too!This ain't an uma, it's a UMA!!