Chrétien de Troyes would appear to mention three Yders in Erec and Enide. I doubt that Yder of the Dolorous Mount, named among Arthur’s knights in the list beginning in line 1691, should be identified with Yder the son of Nut.
I see more chance that he and King Yder are the same, and that the exigencies of Chrétien’s original verse account for the differences in nomenclature.
Source
Yvain, or Le Chevalier au Lion | Chrétien de Troyes, late 12th century