In Pa gur, there is a mention of Arthur fighting “dychryn Cŵn,” meaning “the terror of dogs.”
“What man, from the time of the beginning,
Pa gur, o vynych daear,
Pan vynnwys yscwyd Caledfryn,
Dygen Cŵn a chwanwn
O’ amneint a gwnaethant brynnawn.
When the shield of Caledfryn was wielded,
Ever before bore dogs with chains,
From the high lands they made a tumult.”
Another passage says:
On the mount of Eidyn, they [Arthur and presumably Cei] found with Dog-heads; by the hundred they fell.
They are not necessarily monsters. H. Butler has advanced the theory that they were related to the Conchind (Dog-heads), a legendary people who ruled Ireland. There may be some connection with the Cunesioi, a tribe whom Herodotus places beyond the Celts in the Iberian Peninsula, and the Concani who, according to Horace, lived in Spain.
Pa gur yv y porthaur | Poem 31 of the Black Book of Carmarthen, probably c. 1100