Elizabeth or Joan was an illegitimate daughter of Henry Beauclerc, but her mother is uncertain. It is probable that a daughter of Henry married Fergus. Whether Elizabeth was really married to Fergus of Galloway, or whether his children had a different mother is uncertain.
Fergus may have married an ilegitimate daughter of Henri Beauclerc, King Henry I of England. Her name, however, is unknown. One of the candidates is Sibylla, the widow of King Alexander of Scotland, but there is little evidence for this. Another candidate could be Elisabeth; but likewise, there is little evidence. If he did marry a daughter of Henry I, the marriage can be interpreted as part of the forward policy of Henry I in the northwest of his dominions and the Irish Sea zone in general, which was engineered in the second decade of the 12th century. It may have been during this time that Fergus began calling himself rex Galwitensium (King of Galloway). However, while his possible father-in-law lived, Fergus seems to have remained a faithful vassal to Henry.
Alison Weir, Britain's Royal Family: A Complete Genealogy (London, U.K.: The Bodley Head, 1999), page 49. Hereinafter cited as Britain's Royal Family.
Elizabeth, natural daughter of Henry I, and sister of Sibilla, who married Alexander I, brother of David." [Ref: Sir Herbert Maxwell, "A History of Dumfries and Galloway" by Sir Herbert Maxwell, Edinburgh, 1896, pp. 47-48]