It's not Perl books that have animals on the cover, it's O'Reilly books (and, actually, only certain kinds of O'Reilly books). Not all O'Reilly books are Perl books and not all Perl books are O'Reilly books :-)

In the case of the Badger book, Andy Wardley (creator of TT and co-author of the book) was sure from the start that he wanted a badger on the front cover (badgers look friendly but are actually very powerful and tenacious) and the editor and the designers were happy to accomodate his request. The colophon of the book contains far more information about badgers.