Within the central game, no. There are new monsters in the bonus dungeons including the bosses, including a boss in the ToF's bonus dungeon whose form differs depending on what type of seal you opened in what order, with each incarnation a different version. Let it be said that the ToF bonus dungeon is a bitch. I've attempted it about six times and failed each time to get the most powerful form. Not because of my lack of skill, the puzzles are just ridiculously annoying.
Incidentally there are no missable enemies in FF1. You can revisit any location and refill your Bestiary.




