Humans are the #1 "Monster" in my book. Do you fight them or talk to them?
Other creatures that have seen use IMC:
Dire Boar, Green Hag, Ogres, Cave Fisher, Kobolds, Spiderfolk, Monstrous Spiders, Shadow Wolves, Shadow Skeletons, Water Elemental Skeleton, many types of normal animals, Orcs, Treant, Stirges, Minotaur, Greywhethers (carnivorous sheep), Goblins, Bugbears, Shadow Orcs, Dire Badger, Vampires, Sentient Skeletal "Druids", Aboleths, Stone Guardians, Dire Corbies, Caryatid Columns, Giant Archer Fish, Giant Pike, Ghouls, Manticore, Allips, Zombies, Wights, Wraith, Satyr, Many Fey of Various Sorts (templated and/or individualized), The Entombed God Baloraz, Grell, Dimensional Shambler, Skitterhaunt Centipedes, Psuedonatural Rats, Stirges, Fire Beetles, Owlbears, Grey Oozes, Gelatinous Cube, Fire-Breathing Giant Bees, Skeletons of Many Diverse Types of Creatures, Displacer Serpents, Grick, Ankheg, Tunnel Worm, Caller In Darkness, Skum, Mist Walker, Gibbering Mouther, Small Monstrous Ants, Smilodon, and a bunch of other stuff that I'll later kick myself for not remembering now.