In Goidelic mythology, Fergus mac Roich (Fergus means "virility") was a courtesan in the court of King Conchobar. After Conchobar killed his nephews, Fergus left the court in protest of his treachery.

He was Cuchulainn's tutor.

He was one of the lovers of Queen Medb. He was referred to as "the great horse" in reference to an enormous phallus.