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 guardsman. 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 and their feats of strength typically surpass even those of Space Marines. Some Ogryns are instead given super-sized shields and extremely thick armor; known as "Bullgryns", these Ogryns serve as living, mobile bunker walls on the battlefield. The idea is that all that armor, behind a huge-ass shield designed to be interlocked with the neighboring shields, and with a big strong Ogryn inside of it, will soak up bullets, lasers and other incoming nastiness that might hit more important things. It's a pretty sound theory. Each Bullgryn also carries an arm-mounted grenade launcher for fighting back with. Though some Bullgryns are instead issued power mauls and deflector bucklers instead of slabshields and grenade gauntlets.
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. Technically this is already Imperial Guard policy to some degree, but only Ogryns actually believe it's literal. The only reason that they aren't used even more is that they don't take well to transports- on top of their size, they have a tendency to get claustrophobic and aren't likely to willingly enter cramped, dark spaces (e.g. the inside of a Chimera).
Additional Material
Gav and Bob - A story about some of the best Ogryns that gave their lives for the Emprah.