Some of O'Reilly's animal books are obvious: the gnu for Gnu Emacs, for example.
Others are more oblique, like the beaver for NT Event Logging, or the Surinam toad for Windows Annoyances.
Yet others are downright subtle.I suspect that the camel on Programming Perl derives from that creature's reputation as a graceless and improbable creature, capable of startling feats of strength and endurance.

Almost all O'Reilly cover images come from the Dover pictoral archive.