[Bahamut2000X]

In the US NES version of IV you could only get Imp as a summon.

However in other non-US SNES versions or later versions of IV that came out to America it was possible to summon bomb, mage, and I believe the last was Cockatrice. All of 3 of which plus Imp can only be obtained from defeating enemies of the appropriate families and hoping they drop the item to teach Rydia them. However they're incredibly rare and up there in rarity along with pink puff tails.