Or Róig, Róeg: the erstwhile king of Ulster in the eponymous story cycle, notably the protagonist of the Táin Bó Flidhais. He was king of Ulster until tricked out of the crown by his wife Ness, in favor of her (but not his) son Conchobar; subsequently he gave his word in surety for the safety of the Sons of Uisliu, so that when Conchobar had them killed, it was he, Fergus, who was shamed, since it was he who had broken his word of protection, and he was driven into exile on this account. With him in exile went many of the best warriors of the Ulaid, disgusted with such treachery: Conall Cernach, Lóegaire Búadach, and Fergus' foster-son, Conchobar's eldest son Cormac Cond Longas, among them. These men formed the troop of the Ulstermen in Exile who came to fight against Ulaid in the great Cattle-Raid of Cooley. In this story, Fergus has a particular and prominent relation to Cú Chulainn, being one of his foster-fathers; the latter often excepts Fergus from performing the various challenges he sets for the Connachtmen, knowing as he does that Fergus could easily repeat them.

The name Fergus literally means »man-strength«, to be understood basically as »virtue« or »virility« (two words whose meanings were less distinct in Latin than they are in English and even less so in Old Irish), and is quite clearly connected to the particular nature of his person and feats: he is enormously strong, immense in size, vastly potent, his appetites gigantic; as for nobility or righteousness, those are virtues of little real importance to the men of old Ireland, but he has them too.