The Imperium is both a diverse and dangerous place, choked with worlds mired with billions of humans. Among all that teeming humanity, and over the course of the many millennia since mankind first stepped out among the stars, it is little wonder that the human race has "devolved" and diverged numerous times into mutation. Many Hive Worlds and similar developed planets have significant mutant populations, the "twisted rejects of humanity warped by chemicals, radiation, or successive generations of inbreeding and genetic impurity". Condemned to a miserable existence as forced labour or culled to “acceptable” levels, mutant populations are "usually" considered an unpleasant problem by Planetary Governors and kept far away from the eyes and lives of its citizens. Mutants are also often seen as "touched by the Warp, or somehow in league with The Ruinous Powers, and thus always considered dangerous and suspicious by Imperial authority, and among the first to suffer the attentions of The Inquisition".
Not all mutants are so scorned or oppressed by the Imperium, and some kinds have even reached a point of sanctioning and induction to the Astra Militarum. These are stable strains of mutants known as abhumans, and primarily comprised of Ogryns and Ratlings. Ogryns are large, bulky humanoids, standing seven or eight feet tall and wrapped in massive slabs of muscle. Despite limited imagination, they make excellent shock troops, where their size and ability to ignore all but the most grievous of wounds allows them to plough into the enemy, leading the way for their comrades. Ogryns are also fantastically loyal and make excellent bodyguards, fighting to the death or standing resolutely over the body of their fallen master like a faithful dog (granted, a seven foot dog armed with a shotcannon). For their strengths, though, Ogryns do have their weaknesses. In addition to their "limited intelligence", which, without proper leadership, can leave them confused or dithering in the midst of battle, they are also adverse to confined spaces, especially dark ones. An understandable trait in such a large creature, the Imperial Guard nevertheless has to transport the abhumans to the battlefield, and it will fall to some unlucky Guardsman to lure the Ogryns into a transport or ship hold (often with the promise of food) and then babysit them through a long and miserable journey.
Ratlings are of a different breed altogether, and where the Ogryn is large and powerfully muscled, the Ratling is diminutive and quick. Only reaching heights of three or four feet, Ratlings make "excellent snipers and infiltrators", their keen senses and small size allowing them to slip past larger foes. Ratlings have their own home worlds, descended from ancient human colonies where they levy troops for the Imperial Guard, and so feel their rightful place among the Imperial Guard just like any other world might. Unlike other worlds, though, "Ratlings are not large or strong enough to make acceptable front line troops" and thus do not comprise complete regiments in the same way as the rest of the Imperial Guard. Instead, Ratlings are used as specialist troops, either as snipers and infiltrators (where their natural marksmanship and skills at stealth outshine ordinary
Guardsmen) or as their other great vocation: cooks.