Found in 8 comments on Hacker News
This is mostly an O'Reilly thing.

https://www.oreilly.com/content/a-short-history-of-the-oreil...

TL;DR:

> Some of the people at O’Reilly were taken aback: they thought the animals were weird, ugly, and a bit scary. But Tim [O'Reilly] got it immediately—he liked the quirkiness of the animals, thought it would help to make the books stand out from other publishers’ offerings—and it just felt right.

They even have a browser which helps you identify the animal:

https://www.oreilly.com/animals.csp

gwern · 2024-03-04 · Original thread
The obvious one is a parrot, but looks like they've already used a lot of parrots: https://www.oreilly.com/animals.csp?x-search=parrot&x-sort=a... Still some room for superintelligent cephalopods https://www.oreilly.com/animals.csp?x-search=octopus&x-sort=... ...
dang · 2022-02-08 · Original thread
Changed from https://www.oreilly.com/animals.csp. Thanks!
rory · 2022-02-08 · Original thread
They claim 1463, yet only show the first 1361! Where have you taken the missing animals, O'Reilly?!

https://www.oreilly.com/animals.csp?x-o=1360

thomasqbrady · 2022-02-08 · Original thread
Fun fact: the author can ask for a particular animal, but it’s not guaranteed. When we were working on our Ember.ha book we–naturallly–asked if we could get a hamster on the cover. They weren’t sure they’d be able to influence the art chosen, but in the end they can as close as they could–a dormouse: https://www.oreilly.com/animals.csp?x-search=Ember&x-sort=an...
bluehex · 2022-02-08 · Original thread
The animal search[1] even returns results for some categories of animals like "crustacean", "bird", and "feline". Nice attention to detail.

[1] https://www.oreilly.com/animals.csp

l9i · 2020-04-09 · Original thread
Confirmed. I was hoping for the platypus but it turns out it was already taken anyway.

You can check out the whole O'Reilly menagerie at https://www.oreilly.com/animals.csp.

(disclaimer: I worked on the book)

pdpi · 2018-02-14 · Original thread
A lot of those are some sort of pun or reference on the topic at hand: The crab as as a reference to Ferris/Rustaceans on the Rust book, spiders on books for "webmasters", the snake on Python Programming, etc. Others were just assigned at random (and proceeded to become indelibly associated with the language, like the perl camel).

A list of the animals, and some history:

http://www.oreilly.com/animals.csp https://www.oreilly.com/ideas/a-short-history-of-the-oreilly...