No other nonhuman source has served as the basis for more metaphors than animals. Speaking of Animals is a dictionary of animal metaphors that are current in American English. It is comprehensive, historical, and metaphor-based. Each entry refers to the other dictionaries that catalog that same metaphor, and the dates of first appearance in writing are supplied, where possible, for both the metaphor and the name of the source. The main text is organized alphabetically by metaphor rather than by animal or animal behavior; all the metaphors are classified according to their animal source in a list at the end of the book.

An animal metaphor is a word, phrase, or sentence that expresses a resemblance or similarity between someone or something and a particular animal or animal class. True metaphors are single words, such as the noun tiger, the verb hog, and the adjective chicken. Phrasal metaphors combine true metaphors with other words, such as blind tiger, hog the road, and chicken colonel. Other animal metaphors take the form of similes, such as like rats leaving a sinking ship and prickly as a hedgehog. Still others take the form of proverbs, such as Don't count your chickens before they hatch and Let sleeping dogs lie. The horse is the animal most frequently referred to in metaphors, followed closely by the dog. The Bible is the most prolific literary source of animal metaphors, followed closely by Shakespeare.