Iolo Morganwg claimed that he had discovered two alternative pedigrees for Crallo, but was torn between which was correct. This may suggest that the documents he had seen were genuine. One indicated that Crallo lived amongst mythical characters of the Roman Era. The other gave him as the son of St. Sadwrn of Llansadwrn and a pupil of his uncle, St. Illtud, at Llanilltud Fawr (Llantwit Major).

There are no historic records of St. Crallo except the place-name Llangrallo. He is generally considered apocryphal.