For those who dont know the GURPS Classic line are PDF reprints of GURPS Third Edition books.
There is typically very little change needed to make most Third Edition material compatible with GURPS Fourth Edition. And Third Edition was a golden age for GURPS with tons of books coming out.
So GURPS Bestiary contains a lot of animals and creatures from various cultures. All of which could be used as is or minor attribute/stat tweeks if you want them to be realistic.
It also includes some notes on tactics and using them.