"The tail I could live with. But the fleas, they're murder."
Sirius Black about his dog-Animagus form[src]

Flea is the common name for insects of the order Siphonaptera. They are wingless insects with mouthparts adapted for piercing skin and sucking blood. Fleas are external parasites, who live by sucking the blood of mammals that include including bats, humans and birds.

James Potter once suggested that Sirius Black should be a dog for permanent. Years after Sirius commented to James' son, Harry Potter, that he could live with the tail but that the fleas were murder.