Mannacus was a 6th-century bishop and monk who is recognized as a saint in Catholic, Anglican, and Orthodox traditions. He is associated with the religious life of Wales, where he served as the abbot at Caer Gybi in Holyhead, Anglesey, working alongside Saint Cybi. Some traditions suggest he was a hermit and a companion to the saint Mybbard. His legacy is closely tied to the Cornish landscape, where he is remembered as a significant religious figure of the early medieval period.