The earliest literary record of the Cath Palug appears in Pa Gur yv y Porthaur ("What man is the gatekeeper?"), Poem 31 of the Black Book of Carmarthen (written before 1250 CE)-one of the oldest surviving manuscripts composed entirely in Welsh and a crucial source for early Arthurian material. The poem takes the form of a dialogue between King Arthur and the gatekeeper Glewlwyd, in which Arthur introduces the heroes of his warband and recounts their monstrous deeds. Among these heroes stands Cei (Sir Kay), Arthur's foster brother, whose battle with the Cath Palug on Anglesey forms one of his proudest achievements. The poem states:

"Cei the fair went to Mona, To devastate Llewon. His shield was ready Against Palug's Cat When the people welcomed him. Who pierced the Cath Palug? Nine score before dawn Would fall for its food. Nine score chieftains…"

Here the text breaks off, the remainder lost to time, but the fragment preserves enough to demonstrate the Cath Palug's terrible power: 180 warriors (nine score) fell to the beast before Cei confronted it. The poem describes Cei's shield as mynud against the cat-interpreted as "polished" or "smooth," suggesting a mirrored surface. This detail connects to a Middle English tradition in the Lambeth manuscript where Arthur defeated monstrous cats by raising a polished shield, causing the creatures to attack their own reflections. Whether Cei slew the Cath Palug or merely drove it off remains ambiguous-the fragment's incompleteness has left scholars debating whether the beast survived its encounter with Arthur's foster brother.

The Cath Palug's aquatic nature constitutes one of its most distinctive characteristics. Unlike ordinary cats, which famously hate water, the Cath Palug was semi-aquatic, haunting lakes, rivers, and the sea. This water-association carried across to the French tradition, where the Vulgate Estoire de Merlin places Arthur's battle with the monstrous cat (here called the Devil Cat of Lausanne) near Lake Bourget in the French Alps. According to this version, the cat originated as a black kitten caught by a fisherman who had broken a vow to God-keeping the first fish he caught for himself rather than offering it to the divine. The kitten grew into a monster that destroyed the fisherman's family and terrorized the region until Arthur confronted and slew it. French toponymy preserves this memory in names such as Col du Chat (Cat's Neck), Dent du Chat (Cat's Tooth), and Mont du Chat (Cat's Mountain). The most extraordinary French variant, recorded in the 12th-century Anglo-Norman Romanz des Franceis, claims that the cat Chapalu actually killed King Arthur and usurped his throne-a radical alternative tradition that suggests the Cath Palug's legend once contained a very different ending to Arthur's story.