Nennius, or Nemnivus, is either of two shadowy personages traditionally associated with the history of Wales. The better known of the two is Nennius, the student of Elvodugus. Elvodugus is commonly identified with the bishop Elfoddw of Gwynedd, who convinced the rest of the Welsh portion of Celtic Christianity to celebrate Easter on the same date as the other Catholics in Britain in 768, and is later stated by the Annales Cambriae to have died in 809. This Nennius is traditionally stated as having lived in the early 9th century, and is identified in one group of manuscripts of the Historia Brittonum as the author of that work. The careful scholarship of professor David N. Dumville on this text has instead shown that the manuscripts that make this claim come from an exemplar dating to the later eleventh century, far later than the exemplars of other versions of this manuscript — as well as over two hundred years after this Nennius is supposed to have lived. However, a number of historians still refer to the author of either the original text of the Historia Brittonum, or this specific recension, as Nennius, or pseudo-Nennius.
The other Nemnivus, or Nennius, is mentioned in a Welsh manuscript of the 9th century. In response to the snide accusation of a Saxon scholar that the Britons had no alphabet of their own, this Nemnivus is said to have invented an alphabet on the fly in order to refute this insult. The alphabet Nemnivus is said to have invented is preserved in this manuscript, and according to Nora Chadwick it is derived from the Old English futhark. "Indeed the names given to some of his letters seem to show evidence of an actual knowledge of their Saxon names", Chadwick concludes.
Some conclude that these two figures are the same individual. Others argue that drawing such a conclusion is not warranted, since Nennius, the student of Elvodugus, is arguably fictional, and since the histories of both Wales and Britain over the period in question are quite incomplete.