The County of Fézensac was an eighth-century creation on the north-eastern fringes of the Duchy of Vasconia following Charlemagne's policy of feudalisation and Frankish colonisation. The movement was aimed to offset and diminish the authority of the duke of Vasconia Lupo II after the setback suffered by the Franks at the Battle of Roncevaux in 778 and failure to restrain the Basques, an advance that clearly displeased the Basques. These policies sparked stir on the banks of the Garonne (Count of Toulouse Chorson defeated by Odalric "Wasco"). The county was appointed to a count called Burgund, who judging by his name was not a Basque. Burgund died about 801 and was replaced by a certain Liutard.
Later in 926, after the death of García II, the Fézensac was given as an appanage to García's second-eldest son William. It included the cities of Vic and Auch, its capital, as well as the territory of Armagnac.
From the house of Fézensac descend the Barons de Montesquiou. The comital name of Fézensac was renewed by Louis XVI in 1777 to be carried by the head of the Montesquiou family.