From Wikipedia

Highland Clans

Clan MacGregor

Clan Gregor, or Clan MacGregor, is a Highland Scottish clan. Outlawed for nearly two hundred years after losing their lands in a long power struggle with the Clan Campbell, the Clan Gregor claims descent from Constantin and wife and cousin Malvina, first son of Doungallas and wife Spontana (daughter of a High King of Ireland) and grandson of Giric, the third son of Alpín mac Echdach, the father of Kenneth MacAlpin, the first King of Scotland, a descent which is proclaimed in the motto, 'S Rioghal Mo Dhream, translated as Royal is my Race.

The Clan Gregor is believed to have originated in Scotland during the 800s. The MacGregor's suggest that they take their name from Gregor (derived from the Latin 'Gregorius' and the Late-Greek 'Gregorios' which means "Alert, Watchful, or Vigilant". Gregor is said to be a son of the Scottish king Alpin II Mac Eochaidh and younger brother of Kenneth MacAlpin, the now famous Scottish king who first united Scotland in A.D. 843. Alpin II was the son of Eochaidh VI 'the Poisonous,' High King of Scots, by his marriage to his cousin, the Pictish Princess Royal, and thus had claims to the Scottish and Pictish Thrones.

Alpin was defeated and allegedly beheaded in his attempt to gain the Pictish Throne. His son, Kenneth, was successful, taking advantage of Viking harassment of the Picts from the east. While there is no surviving concrete record of a younger 'Prince Gregor', the Gregg Family website claims that an ancient Latin record of the Alpinian family mentions a Gregor who was a commander in the army of Kenneth Mac Alpin. Kenneth had a least one other known brother, Donald, who succeeded him as king of Scots. Unfortunately, most of the early public records of Scotland were destroyed by order of the English King Edward Plantagenet, during his occupation of Scotland at the end of the 13th century.