Why do programming books have animals on the covers?

The source, https://www.oreilly.com/content/a-short-history-of-the-oreil... , is rather more interesting.

What was the first animal book? The Xlib reference manual (1990) does not use it. https://archive.org/details/xlibrefmanv115ed02nyemiss

Programming Perl (1991) used a camel. https://archive.org/details/programmingperl000wall

Ahh, Learning the vi Editor (1990) had an animal. https://archive.org/details/learningvieditor00lamb_0

So did Using C on the UNIX system : a guide to system programming (1989), https://archive.org/details/usingconunixsyst00curr . With a lion.