A Welsh warrior who, while hunting in the mountains of Arwystli, went insane from eating poisoned apples that had been intended for Merlin.
He and Merlin (who had gone insane for other reasons) were both cured at a fountain in the forest of Caledon. Maeldinus then chose to live in Caledon with Merlin, Taliesin, and Ganieda.
Source
Vita Merlini | Geoffrey of Monmouth, c. 1150