I hope it's not Bahamut Zero. Otherwise, I messed up. Okay, guys! Now get even more specific than that! You're close. It's obvious how to be more specific, so I'll just say that I hope there isn't any summon materia in the North Crater or Midgar, b/c if there is, I messed up.

EDIT: Uh oh. Is there one in the Underwater Reactor? I don't think so, but if there is, uh, pretend it didn't exist!

Don't forget about the first part of the riddle. That's another route you can take. The route you are on doesn't have much distance left on it, and if you figure out the first part of the riddle, the rest of the distance to the answer is pretty short. Being part of the 2nd found of something different implies that the answer is right in front of you if you figure out the first part of the riddle.

EDIT: whoops, I should probably add also that if you use the first part of the riddle, it's not the second thing literally, but the second type of the thing that's not a summon materia.