It happens… that cowardly men run wild and lose their wits from the dread and fear which seize them. And then run into the wood away from other men and live there like wild beasts, and shun the meeting of men like wild beasts. And it is said of these men that when they have lived in the woods in that condition for twenty years, then feathers grow on their bodies as on birds, whereby their bodies are protected against frost and cold, but the feathers are not so large that they may fly like birds. Yet their swiftness is said to be so great that other men cannot approach them, and greyhounds just as little as men. For these men run along the trees almost as swiftly as monkeys or squirrels
Speculum Regale, old Norse, 13th cen.