So, you're a Chaos Champion. You want to do a bit of conquering and rampaging. You're gonna need an army. It's all well and good having your big meaty, armoured retinue but there's not a lot of them. The beastmen are fairly useful but they smell awful and you can never quite tell if they've understood your orders. As for the minotaurs and ogres....sheesh. No, you need some human followers of the dark gods. Some useful cannon fodder who can actually hold a conversation as well as take part in a summoning or two. You need some Chaos thugs!
These illustrations come from the Chaos Thug entries from Warhammer armies and Slave to Darkness. There were three levels of Human mortal in the chaos lists, the highest being the Warriors, with the Marauders beneath and at the bottom of the pile are the Chaos Thugs. The lists contain options for both mounted and foot. Unfortunately for us citadel never made any mounted thugs. We can only imagine what they would have been like.