» Tue Feb 01, 2011 10:21 pm
Malacath began as the Aedric Trinimac, and was fundamentally altered completely outside the bounds of his original sphere by the acts of a Daedra, Boethiah. His nature, along with Meridia's, is somewhat different and unpopular among the other Daedra due to this, and gives certain odd aspects to him. This resulted in, among other things, the conversion of Trinimac believer-but-not-devout Altmer into the Chimer (before the color of their skin was altered at a much later date) under Boethiah's power, while those who remained devout to Trinimac http://www.uesp.net/wiki/Lore:The_True_Nature_of_Orcs. Exiled isn't really the right word -- Lorkhan was exiled by the Aedra by being torn apart and forced into Nirn while retaining (at least in part) his previous attributes as a Trickster/Mentor/Inspiration aedra). Trinimac was completely converted from an aedra of the Mer warrior spirit, to a Daedra-like being of outsiders, the sworn promise, and the bloody oath (and probably strength), and has access to both the Mundus and Oblivion as much as any other Daedra.
This story dates back to at least Morrowind, and there's not much mention of Malacath (or the Ogrim, or the Ogres) before then, outside of just naming him.
Their origin's probably the same as any other minor beastfolk -- either unknowable, or a side-effect of the Mythic Era and the Aedra's formation of the world.