Symbolism, Imagery, Allegory
Elphaba's familiars are like a bunch of pet-store rejects or characters in a Stephen King novel: a swarm of killer bees, a talking but taciturn monkey, some ominous crows, and an anti-social dog named Killjoy. Of course these are a Witch's familiars, so the creepy and weird factor is probably fitting. And the Witches in this book are definitely not of the Hermione Granger variety.
Elphaba's familiars demonstrate how appearances can be deceiving, which is a running theme throughout the book. All of these animals seem violent and mean, but they actually prove to be loyal, helpful, and even loving around Elphaba and her family. Elphaba definitely has a gift for dealing with animals, and it's no accident that her major political crusade involves animal (and Animal) rights.