Fergus is a playable character in Fire Emblem: Thracia 776. He is a mercenary of mysterious origins, with rumors saying that his father is Beowolf (meaning he is possibly the half-brother of Diarmuid and maybe Nanna) and that his mother is a noble from Conote. He was thrown into a Manster prison for beating up soldiers that were harassing Karin.
Strangely, Fergus is one of the only two characters who can use the Beo Sword, with the other being Diarmuid (while Diarmuid's sister Nanna cannot wield it). This has given even more fuel to the theory that he is Beowolf's son.