|GW Catalog circa 1988|
In the background, they were smaller, one-man counterparts of the much larger Titans which were crewed by many more people. Originally, they were derived from Eldar technology and were copies of Eldar Knights, used by Exodites to assist in the herding of large, dinosaur-like reptiles they used as livestock. Human Knight Titans were used in the same fashion, and were assimilated by the Imperium's armies whenever they were encountered.
Outside the tabletop games, Knights have not appeared much in other media. The 2008 Black Library book 'Mechanicum' features several Knights from a Mars based legion set at the outset of the Horus Heresy. Knight Paladins were secret units in PC game Epic 40,000 Final Liberation.