As I mentioned previously, The Crypts & Things monster section is quite evocative, but is not comprehensive when it comes to ordinary creatures and the like. A terrific resource for such things is the Swords & Wizardry Monster Book by Matthew Finch. It includes hundreds of monsters - both fantastic and mundane- some may not be a good fit for Zarth, but I could see using many of them in a C&T/S&S game. What it does have that I really appreciate are listings for things like wild animals (both giant and normal) as well as mounts (horses).The stats jibe nicely with C&T monsters (just remove the Save stat) and the book is available on Lulu in pdf or print on demand.
I highly recommend taking a look.