An Atavism is one of the manifest Beasts of the Primal Wilds, emboying the Arcanum of Life, that can be called down by the use of Supernal Summoning.
Atavisms represent animals or plants -some that trod the Fallen World millions of years ago, others that never have- and may resemble dragons, unicorns or other mythical creatures. Does this mean that such creatures ever lived in the Fallen World? It’s hard to say. The Primal Wild is a realm of spirit and flesh, and the line between the two blurs considerably in the Abode of Beasts. A given Atavism might have no fossil record, but that doesn’t mean it was never a spirit — and if it’s a spirit, in the Primal Wild, it has a body. Most are possessed by a feral cunning and some are even capable of coherent speech.
Mages who summon them can learn many things about extinct creatures from them and use parts of an Atavism for extremly efficient poisons or remedies.