The Star Beast

The Star Beast is a 1954 science fiction novel by Robert Heinlein about a high school senior who discovers that his late father's "pet" is more than it appears to be.

John Thomas Stuart, one of whose ancestors brought "Cuddlepup" home from an interstellar voyage, has a problem. The pet he inherited has grown to gargantuan proportions; it ate a used Buick and (still worse) destroyed a neighbor's flowers. His mother wants him to get rid of it, and a court orders it destroyed.

Johnny's only alternative is to sell his pet to a museum (which would study it to death, if they didn't dissect it immediately). So early one morning he runs away to the hills, riding on his talking pet's back. His girlfriend joins him and suggests bringing the beast back into town; they could hide it in a neighbor's greenhouse.

The rest of the novel describes the discovery of the alien pet's true nature, and the diplomatic repercussions of this fact.