The earliest mention of Catigern appears in the Historia Brittonum, written in the 9th century and attributed to the monk Nennius. Chapters 43-45 say that the British king Vortigern had been appeasing the Saxons, and his son Vortimer rose against the enemy and engaged them in four battles. At the third of these, the Battle of Epsford (Aylesford), Catigern fell, as did the Saxon leader Horsa. Chapter 48 also mentions Catigern as Vortigern's second son after Vortimer and before Pascent and Faustus, and reiterates that he died in the same battle as Horsa. Neither mention is clear as to which side Catigern was on, but context implies he fought alongside his brother against the Saxons. The Anglo-Saxon Chronicle describes this battle and the death of Horsa in the entry for 455, though it does not mention Catigern.

A Catigern, here Cattegirn (Celtic cato- "battle", tigerno- "lord"), also appears in the Harleian genealogies. This Cattegirn is included towards the head of three of the pedigrees of rulers of Powys. In each of these, however, he is said to be the son not of Vortigern, but of the legendary Powys ancestor figure Cadell Ddyrnllwg. This Cadell is known from the Historia Brittonum, which says he was a servant who had been converted by Vortigern's enemy Saint Germanus of Auxerre, and thereafter became a king whose descendants ruled Powys through the centuries. However, one of the genealogies from Jesus College MS 20 refers to a "Cadern", father of Cadell and the son of Vortigern.