The MacDonalds were a strong force in north east Scotland. So much so, that up until the Robertsons started, they did not recognize the Scottish monarchy's authority over them. At the point we are interested in, William the Lion was King of Scotland. But Angus Mor (c. 1250-1296) considered himself Lord or even King of the Isles. Angus Mor's father was Donald of Isla. The gaelic term "Mac" means son, so we see how the name MacDonald started. He was the first of his family to acknowledge the Scottish King, but most likely because King Edward I of England was overlord. This Angus Mor did only toward the end of his life in 1296.
Angus Mor had 3 sons. His eldest son Alexander, who was banished for his opposition to Robert the Bruce, succeeded him. Alexander was succeeded on his death in 1303 by his brother Angus Og. According to this first theory, the third son was Duncan the Fat. Duncan in Gaelic is "Donnachie" from which the Robertsons evolved.