In Geoffrey of Monmouth’s chronicle, a son of King Morvid who succeeded his brother Gorbonian to the throne of Britain in the third century BC.
Unlike his brother, he was a tyrant. His nobles eventually deposed him and installed his brother Elidur as king. Elidur reconciled with Arthgallo and forced the nobles to accept him as king again.
Arthgallo reigned for another ten years, this time in justice. Elidur succeeded him again when he died. Arthgallo’s sons, Margan and Enniaun, were both later kings.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138