Eggman can be summoned legitimately in all versions of the game by using the Magic Lamp. Once Chocobo has been summoned 20 times, Eggman will be summoned instead until the Magic Lamp is recharged. When summoned, Eggman's sprite does not actually appear in both the Japanese version and the English version, and will try to use Egg Chop and fail, displaying the message "Too far away!" or "Can't reach" in the PlayStation version.
If the Eggman is hacked into the game as an encounterable enemy, its name will appear blank when fighting it due to not having a name in the game's code. It will take on the stats of whatever enemy it is replacing, including attacks, HP, and items dropped. The video below shows a battle against a set of Eggman enemies that have replaced the Grenade enemy.