Domnall mac Causantín, anglicised as Donald II, was King of the Picts or King of Alba in the late 9th century. He was the son of Constantine I. Donald is given the epithet Dásachtach, "the Madman", by the Prophecy of Berchán.

Causantín mac Cináeda was a king of the Picts. He is often known as Constantine I in reference to his place in modern lists of Scottish monarchs, but contemporary sources described Causantín only as a Pictish king. A son of Cináed mac Ailpín, he succeeded his uncle Domnall mac Ailpín as Pictish king following the latter's death on 13 April 862. It is likely that Causantín's reign witnessed increased activity by Vikings, based in Ireland, Northumbria and northern Britain. He died fighting one such invasion.