Aeon is one of the rarest enemies in Final Fantasy III. This Seeking Tech guide will show you the best location to find him.
Aeon will spawn in the world map area close to the Ancients’ Maze and Crystal Tower final dungeons.
The best spot is in the small in-between area in between these two dungeons. Here, you need to run back and forward in the grass sections.
Eventually, you should be able to run into Aeon.