Amustans, Amustant
In Lancelot of the Laik, the name given to the character unnamed in the Vulgate Lancelot who, during Arthur’s war with Galehaut, upbraids Arthur for his failings, and instructs him on how to be a virtuous and noble king.
His name may have been inspired by Amustan in the Vulgate romances.
Source
Lancelot of the Laik | Late 15th century