Custennin, Kustenin, Kustennin, Mustennin
The early Welsh form of 'Constantine', used to designate Constantine, grandfather of Arthur, and is found in this form, or its alternative Kustenin, in the paternal descent of Arthur from Llyr in the Mostyn MS 117 and in the Welsh Bonedd yr arwr.
The curious variant of Mustennin appears in one version of Arthur's paternal pedigree found in the Bonedd yr Arwr. It seems most probable that this variant is simply a transcriptive error.
Custennin | The Legend of King Arthur
Mustennin | The Legend of King Arthur