As a long time fan of Vampire: the Masquerade, it always amused me when people said that was the first game to focus on monsters. Monster! Monsters! of course predated it by nearly 15 years.
Another Zanny brainchild of Ken St Andre, Monsters! Monsters! follows that same simple systems as Tunnels and Trolls - a d6 pool and summation mechanic with a xd6 skill mechanic focused primarily on a character's attributes. The rules are fast and easy, and can be learned in about 5 minutes, and the character sheet fits on an index card.
Monsters! Monsters! however, takes the game in totally new directions than T&T. There are dozens of monster kindreds to play as, with their own unique rules and modifiers. Whereas other roleplaying games focus on rules complexity, M!M! tries to make the game interesting with tons of variety to supplement the simple rules. Additionally, M!M! 2nd edition is only 67 pages long, introducing some rules clarifications and additions from Deluxe Tunnels & Trolls without bogging down the game with setting or minutae. If you do want some setting material for the world, you can check out the comparatively massive dT&T.
Finally M!M! 2nd edition finishes off with a basic adventure in a dwarf hold. It's a fun, quick romp with a terrifying adversary at the end.