Custennin
According to the pedigree of Anlawdd that has been postulated by T.W. Rolleston, Custennin was the son of Anlawdd and brother of Yspaddaden (Ysbadadden) and Goleuddydd.
The same author prepared a genealogy for King Arthur in which Custennin appars to be a simple variant of Kustennin.
Constantine III | The Legend of King Arthur
Kustenhin | The Legend of King Arthur
Mustennin | The Legend of King Arthur