Ancient historians and chroniclers were fascinated by stories about weird tribes. The two world-spanning histories that come down to us from antiquity, Herodotus's Histories and Pliny the Elder's Natural History, are replete with tales of bizarre tribes that were similar to humanity but not quite the same. These are all great material for weird and somewhat savage characters in roleplaying games. I've included some images of these types from the Nuremburg Chronicle.
cynocephali but they are hardly the only ancient tribe that was generally like humanity, but with a small twist or two. The Arimaspoi pictured to the left, for instance, are human-sized but have only one eye; one has to imagine that their skill with missiles is not significant. In Herodotus, they were the enemies of gold-hoarding gryphons, and lived outside of the idyllic Hypberborea. They make a great substitute for cavemen or similar tribes in mountains or caves.
What is strange is whether or not the headless men are supposed to be intelligent; the name may literally mean "brainless," implying a frightful savage type.
Each of these tribes, and several other types (troglodytae who went from cave-dwelling humans to inhuman monsters, myrmidons who were supposed to have sprung from ants, amazons and others) were always off in the furthest known stretches of land - north of the barbarian tribes you knew in Europe, south of Ethiopia or east of India. They are probably based on innuendo, rumor and ignorance, and grew in the imaginations of travelers.
Once expectations have been upset, of course, the door is also open for NPC members of these tribes to advance in various classes and levels. A one-eyed sorcerer or a fierce headless warrior can be a fascinating foe or ally — obviously depending on circumstances and alignment. Some can also have various types of shamans, witch-doctors and the like, as tribal types are inclined to.
You can see all of the Nuremburg Chronicle's odd people at Wikimedia Commons, and there is a terrific resource for Greco-Roman mythological tribes over at the Theoi Greek Mythology site. And drop me a line if you use these types in your game.