Beli Mawr was an ancestor figure in Middle Welsh literature and genealogies. He is the father of Cassivellaunus, Arianrhod, Lludd Llaw Eraint, Llefelys, and Afallach. In certain medieval genealogies he is listed as the son or husband of Anna, cousin of Mary, mother of Jesus. According to the Welsh Triads, Beli and Dôn were the parents of Arianrhod, but the mother of Beli's other children—and the father of Dôn's other children—is not mentioned in the medieval Welsh literature. Several royal lines in medieval Wales traced their ancestry to Beli. The Mabinogi names Penarddun as a daughter of Beli Mawr, but the genealogy is confused; it is possible she was meant to be his sister rather than daughter.

Cassivellaunus was a historical British military leader who led the defence against Julius Caesar's second expedition to Britain in 54 BC. He led an alliance of tribes against Roman forces, but eventually surrendered after his location was revealed to Julius Caesar by defeated Britons.