The Animal Desk Reference by Melissa Shelton is a reference guide for using essential oils on animals. Edit
Unfortunately, it is brand specific, explaining the uses of single oils and Young Living's oil blends, which can be frustrating if you do not have access to YL's blends. It can also be vague about how to apply the oils for each remedy.
However, it is very extensive and at minimum, has always given me a good starting point for a remedy I may not have otherwise known was possible. Like many guides, it is split up into sections for each oil/oil blend, individual health concerns, animal-specific application methods, and safety. Its main animal categories are "birds & exotics", cats, dogs, horses, and "larger animals", with sections for reptiles, fish, rabbits, ferrets, chinchillas, guinea pigs, elephants, hippopotamuses, hedgehogs, primates, rodents, insects, turtles, horses, squids, octopuses, zoo animals, and even unicorns! Being that extensive, I understand how parts might be vague.