I always heard the stories where King Arthur was taken to Avalon after his death, and was supposed to sleep until England needed him. However, there are also stories where he's still hanging around as a bird - either a raven or a chough. Choughs are recognizable by their red beaks and legs.
The tradition was mentioned as early as in Don Quixote. The first detailed examination was the 1903 book Popular Romances of the West of England. In this was gathered traditions of Arthur appearing as a raven, crow, or chough, particularly in Cornwall. There's a really good rundown of all this at "'But Arthur's Grave is Nowhere Seen': Twelfth-Century and Later Solutions to Arthur's Current Whereabouts."
I first stumbled across this Arthurian tidbit in a story about a pixie who loses his laugh and regains it with the help of the chough King Arthur. You can find this tale as "The Adventures of a Piskey in Search of his Laugh" in North Cornwall Fairies and Legends by Enys Tregarthen (1906), or "The Pisky Who Lost His Laugh" in Traditional Cornish Stories and Rhymes. by Donald R. Raw (1971).
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Researching folktales and fairies, with a focus on common tale types.