The following fiends can be unlocked through various means by capturing other fiends for the Monster Arena.
Unlock two Area Conquest fiends.
Unlock two Species Conquest fiends.
Unlock six Area Conquest fiends.
Unlock six Species Conquest fiends.
Capture one of each fiend from every area in Spira.
Capture five of each fiend from every area in Spira.
Capture two of each underwater fiend in the Mt. Gagazet caves.
Capture ten of every fiend across Spira and defeat all of the other creations in the Monster Arena (including Original, Area and Species).