other appearances

While Diablos has never been encountered in any other Final Fantasy with the same name, we can assume that 'Diablo' from FFVII and 'Devil' from FFIII are the same creature.

I saw a picture of Diablo from Final Fantasy VII and he appears to be a dark creature with dragonlike qualities.

As for FFIII, the quality of pixels and because I have never played the game, I cannot say.