Horses in the 41st Millennium are radically different compared to their ancient descendants first domesticated on Terra. Natural selection and mutation has given rise to horses able to survive in alien atmospheres and consume exotic fodder. Those bred for riding can run at speeds faster than their ancient cousins over much greater distances; draught horses, meanwhile, are grown to such sizes that they would dwarf a Shire or Percheron.
Besides their natural breeding, war-horses used by Rough Riders and other mounted formations are often modified further to increase their combat potential. For example, the Death Riders of Krieg ride on genetically-modified creatures grown in vitagenic cloning vats beneath the surface of Krieg. These horses are able to effortlessly leap over trenches and power through thick mud, assisted by splayed feet for better traction, and can sustain injuries which would kill a lesser creature. Larger than normal horses and nearly hairless, they are psychologically more aggressive and given combat drugs to make them fearless in the heat of battle. However, their volatile body chemistry requires constant monitoring by the rider, with additional chemicals administered to keep it stablised.