Magic.I thought Vellum might be the Caterpillar. A non-Hookah-smoking earth-digging Caterpillar >.> And maybe Von Bon's the Rabbit, but like... as a chicken. I don't know ._.
They're just designed to look like Alice in Wonderland characters. Crimson Queen is the Queen of Hearts, Pierre is the Mad Hatter, and Vellum is the Jabberwocky. People speculate about Von Bon being the Mad Hatter, but I personally think he's the Dodo Bird. There's no particular lore-based reason, just design.