Poisonous animals are widely distributed throughout the animal kingdom; the only major group that seems to be exempt is the birds.

Zootoxins can be divided into several categories: 

(1) oral poisons—those that are poisonous when eaten; 

(2) parenteral poisons, or venoms—those that are produced by a specialized poison gland and administered by means of a venom apparatus; and

(3) crinotoxins—those that are produced by a specialized poison gland but are merely released into the environment, usually by means of a pore.