Mythical king of early Ulster, probable double of the better-known Fergus mac Róich, whose fantastic story, Echtra Fergusa maic Léite [The Saga (or Adventure) of Fergus mac Léti] exists in two widely divergent versions, one from the 7th or 8th century, and a burlesque, Rabelaisian one from the 13th. Suffering a disfigurement from fighting a seamonster, Fergus was disqualified for his throne, even though his subjects revered him. The narrative includes the earliest portrayal of the leprechaun, quite different from its appearance in modern popular literature.
When Eochaid Bélbuide is slain by the men of Conn Cétchathach [of the Hundred Battles], Fergus mac Léti, Eochaid's protector, demands compensation. He accepts a parcel of land and the mother of one of the assassins, Dorn, whom he treats as a menial. Shortly after this he makes a journey by the sea where there are water sprites, each with a small body, lúchorpáin [leprechaun], who relieve him of his sword and carry his body to the water. Fergus awakes when his foot touches the water, and he seizes the sprites by the neck, demanding three wishes of them: that he be given the power of swimming under water in seas, pools, and lakes. The little men grant him this three-part wish by one of two means, either magical herbs in his ears or by winding a waterproof tunic over his head. But he will not be allowed his new power at Loch Rudraige [Dundrum Bay, Co. Down] in his own country. Despite this prohibition, Fergus tries, some time later, to swim under the waters of Loch Rudraige, where he encounters the fearsome monster muirdris. At the sight of this creature, which alternately inflates and deflates itself like a bellows, Fergus's mouth is turned to the back of his head and he escapes to the land. Fergus's charioteer sees his transformation and notifies the wise men of the capital, Emain Macha, who had now to decide the kingship. They esteem Fergus but feel they must follow the requirement that no man with such a blemish can be king. Their solution is to have only sympathetic nobles visit the palace and to banish all mirrors so that even Fergus will not know, solving the dilemma for seven years. But the enslaved noblewoman Dorn feels less compunction. One day when Fergus beats her with a whip because he thinks she is washing his hair too slowly, she taunts him for his deformity, whereupon he cuts her in two. Fergus then returns to Loch Rudraige to deal with the muirdris, roiling the water for two days and turning it red with blood. At the end Fergus emerges victorious with the head of the monster, but soon after drops dead from exhaustion.
The burlesque 13th-century version expands the roles of sprites or leprechauns, introducing their king, Iubdán, his queen, Bebo, and their court poet, Eisirt. Fergus again encounters a sea-monster, now called sínach, in Loch Rudraige, which turns his mouth to the back of his head, disqualifying him from the kingship. His secret is revealed by his wife, when they quarrel over the use of a bath stone. In a second encounter with the monster, Fergus slays it with his sword caladhcholg (see CALADBOLG), but not before it has torn out his heart.