The Legend of Sawney Beane
In my current researches, dealing with all manner of British cannibalism and cynocephali, I find two interesting things. The main impetus of my research, at least at first, was to provide a Celtic folkloric or even cryptozoological origin for the character of Sawney Beane. I've found some possible derivations of the name and his attributes.
1) A host of creatures called "Cynbyn" (dogheads) took part in the Battle of Tryfrwyd, and the Gwrgi Garwllwyd of my research was said to be their chief. Many of the Cynbyn were slain by Bedwyr (later remembered in Arthurian tradition as Bedivere). The battle took place at Mynydd Edyn (modern Edinburgh). Much of southern Scotland up to the Firth of Forth were Celtic (which, on the main isle of Britain, were mainly culturally Welsh) kingdoms, Edinburgh particularly was the kingdom of Gododdin.
The Triads of Wales mention that all manner of British outlaws swore to Gwrgi and served him. The Leges Edwardi Confessoris also mention that a custom among the Saxons, at least, was to refer to an outlaw as a "wolf-head", as he could be freely killed like a wolf. In Gododdin and surrounding kingdoms in particular, I wonder if a similar custom was transferred to the Celtic people, and that therefore these Cynbyn were not true cynocephali even, but references to outlaws. Perhaps Bedivere killed a legion of rogues.
Anyhow, given the region of Mr. Bean, sorry, Beane (hah! the cannibalistic tendencies of Rowan Atkinson are revealed), I wonder if Sawney Beane was a later corruption of Cynbyn. After all, in addition to a cannibal, Sawney and his family were bandits, weren't they?
2) I also found another reference to an Irish tale called Buile Shuibhne, or The Madness of Sweeney. It describes a fellow named Sweeney, chief of the kingdom Dál nAraidi in Ulster. He assaulted a bishop, was stricken mad, and became a poetry-writing woodwose. Different ending, to be sure, but I wonder if this could somehow be the origin, given the Irish kingdom of Dal Riada which existed on the islands between Galloway and Northern Ireland.
I also note that one king of Dál nAraidi was an Áed Dubh mac Suibni, who given the name may or may not be the "Sweeney" above. He killed Diarmait mac Cerbaill, the last High king of Ireland; he bizarrely became a priest, and was cursed by our old Celtic friend St. Columba: "will return as a dog to his vomit; he will again be a bloody murderer".