Ogryn
Back in the days before the Age of Strife, humanity had whole planets dedicated to housing criminals, a fair number of them were barely habitable heavy gravity worlds for some reason. Then the Eldar started creating Slaanesh and society fell apart and these prisons were left isolated. The convicts then got out as things broke down and proceeded to go nuts in orgies of violence and sex. For the next few thousand years these louts ate food and drank water and moonshine with large amounts of mercury and lead in them, were subject to unhealthy doses of radiation, engaged in plenty of incest and leadership (and the ability to acquire mates and spread one's genes which goes with it) was determined by who was the best at beating others with lead pipes. By the time the Emperor got around to reunifying human civilization, there were dozens of planets full of dim witted, durable, strong, three-meter tall dudes called Ogryns.
Though the Imperium of Man typically destroys anything too divergent from the human form, the Ogryns DNA were considered stable enough to classify as a new evolution, or Abhuman. The Imperial Guard has been using Ogryns as heavy shock troopers for millennia, helped by the Ogryns' massive strength, iron-hard loyalty to the Emperor, and literal mindedness. Particularly intelligent Ogryns are given genetic enhancements called Bio Ogryn Neural Enhancement (BONE) and become BONE'EADs, effectively squad sergeants.
Ogryns are usually used as close quarters soldiers, they are given overgrown heavily built automatic shotguns with big bayonets as well as overgrown grenades. Since they are stronger they can also wear more armor than a regular human guardsmen. While your average guardsman may not be too hot in hand to hand combat with an Ork, an Ogryn can give said Ork a run for his money crush Orks into pulp. Perhaps the most appealing aspect of commanding Ogryns is that their loyalty to the Emperor of Mankind is firmer than that of a Space Marine. The Ogryns believe that the Emperor had personally issued them every order (being worked down from the chain of command) and as a result they will follow orders without question to the best of their ability.
Additional Material
Gav and Bob - A story about some of the best Ogryns that gave their lives for the Emprah.