Cath Palug

According to this source, it started life as a kitten (lit "whelp"), given birth by the great white sow Henwen at the black rock in Llanfair [cy].

[d] There the kitten was cast into the sea, but it crossed the Menai Strait and was found on Ynys Môn (Anglesey), where the sons of Palug raised it, not realizing the cat was to become one of the three great plagues of the island.

[7][8] Cath Palug was fought and slain by Cai (Kay), or so it is implied, in the incomplete Old Welsh poem Pa Gur yv y Porthaur found in the Llyfr Du Caerfyrddin (The Black Book of Carmarthen, written before 1250).

[12] This description coincides with the Middle English story in the Lambeth manuscript,[13] in which Arthur raises a shield (presumably mirrored) causing the cats to attack their own shadows reflected in it.

His defeat is noted in several romances that are essentially non-Arthurian, but can be viewed as a French joke against the English, although some researchers believed some genuine tradition of an alternative death of Arthur.

The creature believed to represent the Cath Palug is a spotted feline, seeming to attack King Arthur (labeled rex Arturus) mounted on some horned animal, wearing a crown, and holding a club (or sceptre).

[4] A French original is thought to have existed[30] to the fragmentary, Middle German poem Manuel und Amande written between 1170 and the beginning of the 13th century.

According to the summary given by Emile Freymond [de] (and by Gaston Paris), Galeran of Brittany beats his German opponent Guynant, and the latter tries to rile up the Breton by repeating the contrueve ('idle lie') that the great cat killed Arthur in a pitched battle.

[41] An extract containing the Chapalu portion was published by Antoine Le Roux de Lincy in 1836;[42][j] Paulin Paris wrote summaries based on a different manuscript.

Although Chapalu was beautiful, his mother could not bear her shame and turned him into a hideously shaped monster, and this curse could only be lifted when he has sucked a few drops of Rainouart's blood.

[44][46] The description of Chapalu after his metamorphosis was that he had a cat's head with red eyes,[47] a horse's body, a griffon's talons (or dragon's feet),[44] and a lion's tail.

[54] According to the Myreur, Ogier was traveling in the year 896 to succor Guillaume d'Orange when he was shipwrecked with his horse Passevent on an isle (Ysle de Trist, nine days sailing from Cyprus), and combats with Chapalu (Capalu).

[52] The decasyllabic Roman d'Ogier (c. 1310) summarized by Knut Togeby, and here too, the capalu was a knight transformed into a lutin by the fées, and he offers to become Ogier's squire.

[56] The modern rediscovery of the Arthurian lore here is credited to Emile Freymond [de], who initially searched for local tradition or onomastics around Lausanne, in vain, then crossing the border into France, and found this spot.

French sources describe it as a small black cat that turns into monster
The Dent du Chat ('cat's tooth') peak of the Mont du Chat linked with the legend in popular folklore