Sword & Sorcerery Studio's Creature Collection has a bunch of baddies of dubious power levels; like a God that's CR 10, you have to deal his full hit points in damage in a single round of combat, or else he just uses his magic item and instantly is restored to full health, or a CR 2 falcon with a vorpal wing blade.
But a few of their CR 1/2 monsters (which were intended to be playable,since they include Humans, Dwarfs, elfs and halflings in the book)
But there are a few quite nice ones: for example:
A CR 4 monster that you can own, the Bottle Imp, a tiny undead who is connected to his bottle. Have the BBEG toss his bottle into melee combat (the bottle is quite tough) and watch as the whisp of smoke attacks for 1d6+4 with a nasty sleeping poison after effect.
At CR 5 is the mighty Hollow Knight, A former paladin whose soul has been fused to his suit of armor, which stands as a Lawful neutral defender. These guys make GREAT plot advancers, and allied NPC's.
Though my favorite are the Ratmen (or Slitheren) They actually seperated ratmen out into different monsters based on what their profession was! (Diseased ratmen, Foamer ratmen Red Witch ratmen)
Bear in mind, this book was published BEFORE the monster manual.