A legendary Welsh lake monster whose name, in modern Welsh, means 'beaver'. This would appear to be consistent with the watery connections usually applied to this mythical creature. The word addanc, when employed as a noun rather than a proper noun, has also been used simply to refer to a spirit that dwells in a watery location, and not an actual creature. In this case the spirit is not an animal, but has a more human appearance. There is, however, no description of the Addanc in either form. The cognate Irish word abhac (dwarf) is derived from ab, modern abha, a river; J. Vendryes claims it originally signified a spirit inhabiting waters.

It was overcome by Peredur (Percivale) who had been given a stone by the Empress of Constantinople which rendered him invisible to it. This afanc devoured maidens and it appears to have had manipulative skills, for it hurled spears and other missiles at anyone who tries to enter its cave.

Arthur himself killed an afanc at Llyn Barfog. Another story tells how Hu Gadarn slew such a creature, but this tale is a late concoction as Hu Gadarn may have been invented to Iolo Morgannwg (1747-1826), who claimed that Hu Gadarn had led the Britons to Britain from Sri Lanka.