[JET73L]

that's what an esper is, but how is it? for example, it is a soloidification of Mist, but the Mist is a material that is more susceptible than most matter to field forces such as thought. The espers could be solidifed by philosophers or alchemists determined to think of the ultimate warrior, and once you defeat it, you have gotten some idea of how it fighs and what it's attacks are. Thus, you can summon it yourself, though it will take a while before you can get used to the intricacies of the esper, which is why the one you summoned isn't as strong as the esper you had to fight to get it.

Like that.

Plus: the point of the phoenix down/suspended animation was that in battle, there would be a way to keep a KO'd character from being attacked by monsters until they were revived. Although I like the chocobo theory, and the nuts and greens could contain different development chemical things.