For several eons there was a season of unicorns among the Lords of Chaos -- a kind of in-joke or competition wherein random mortal creatures were selected for unicorn-hood. That is, the perfect metamorphosis from, say, the form of a rabbit to that of a full-blown, sometimes anatomically incorrect mono-horned beast. They called this game Unicorning.
Each unicorn was absurd in its own way, with absurdity being the Chaos Lords' sole aesthetic, so that a wild variety of the beasts began to accumulate in remote forests and in certain secluded glens.
Afterwards the Lords killed them all. All save the most beautiful -- a nine-legged hermaphroditic horse-thing the Jale God called Atanuwe -- an old word meaning something akin to "primal nonsense."
He bred Atanuwe with several human women in his service, and the witch-mothers gave birth to seventeen mule-things with the faces of human men and women. These creatures are called atacorns. They are known to dwell near rivers and underground streams. If witches are sometimes characterized as malicious, the atacorns are regarded as experts in the area of cruelty. They are known in local folklore as child-thieves, cheats, liars, poisoners, slave-traders, cannibals, and occasional usurpers. Several stolen lands make up their hodge-podge empire, wherein Atanuwe is worshiped like a god and eats bread stuffed with the seasoned organs of human virgins.
Such are the doings of Chaos.