Llevelys
King of France, son of Beli the Great, and brother of Lludd, King of Britain, in the non-Arthurian Welsh tale, Lludd and Llefelys.
At the request of his brother, Llefelys helped to rid three plagues from the island of Britain – one of which was a pair of dragons whose screams were heard every May Day, and whose source could not be found. Llefelys caught them and they were buried under the mountain Dinas Emrys and were, presumably, the same dragons that were later uncovered by Merlin.
See also
Lludd and Llefelys | Wikipedia.org
Source
Lludd and Llefelys | 12th or 13th century