Nellisir
Hero
Mongoose released the Monster Encyclopedias I & II near the end of their time with d20. I make absolutely no excuses or justifications for the stat blocks, but many of the monsters were interesting concepts. There was enough range you probably run a campaign without the core books, or come pretty close. A lot of monster books seem to add primarily Rare or Very Rare monsters (ToH2), or they want to do something ALL NEW, which seems to be giving everything names like uthrikcoiwe and blweingwpg (cough Dark Sun cough).