Sometimes described as taking the form of a crocodile, giant beaver or dwarf, it is also said to be a demonic creature. The afanc was said to attack and devour anyone who entered its waters.
Various versions of the tale are known to have existed. Iolo Morganwg, who revived Welsh bardic traditions during the 18th and 19th centuries, popularised a version of the myth that had Hu Gadarn’s two long-horned oxen drag the afanc from the lake, enabling it to be killed. An earlier variation on this had the oxen cast the afanc into Llyn Ffynnon Las (lake of the blue fountain), where it was unable to breach its rocky banks to escape.
In one telling the wild thrashings of the afanc caused flooding which drowned all the people of Britain, save two, Dwyfan and Dwyfach. Another has a maiden who tamed the afanc by letting it sleep in her lap, which allowed her fellow villagers to capture it. When the afanc awoke its struggles crushed the maiden.
Later legends had King Arthur or Peredur slaying the monster. Near Llyn Barfog is a rock with a hoof print carved into it, along with the words Carn March Arthur (stone of Arthur’s horse), supposedly made when his steed, Llamrai, dragged the afanc from the deep.
The afanc has been variously known as the addanc, adanc, addane, avanc, abhac and abac. Several sites lay claim to its domain, among them Llyn Llion, Llyn Barfog ad Llyn-yr-Afanc (the Afanc Pool), a lake in Betws-y-Coed.
Source: Dwyfan and Dwyfach – Wikipedia