Muilearteach (Muileartach, Muilidheartach, Muireatach) is supposed to be the water form taken by the monstrous Cailleach Bheur of Scottish Gaelic tradition, known in the 'Duan na Muileartaich'.
She has various descriptions, as a spirit of tempest it is created by Cailleach with “two slender spears of battle.” Upon her (Cailleach) head the Muilearteach had “gnarled brushwood, like the clawed old wood of aspen root.” [Ross, 233]; on the Sculptured Stones of Scotland it is depicted as a kind of king-fish.
Another description of her runs:
The name of the dauntless spectre Was the bald-red, white-maned Muilearteach. Her face was dark-grey of the hue of coals, The teeth of her jaw were slanting red; There was one flabby eye in her head That quicker moved than lure pursuing mackerel. Her head bristled dark and grey, Like scrubwood before hoar-frost.