Save the game after defeating at least one enemy and you will be able to select this option under 'Bestiary' on the Main menu. This will allow you to see the facts about the enemies you have defeated.

In the GBA Final Fantasy V release there are some bonus Dungeons and Job classes which were unavailable in previous releases of the game.

Unlock Cave:
Get ALL 12 legendary weapons from Kuza Castle.

Unlock Necromancer Job:
In the Sealed Temple defeat Enuo

Unlock Oracle Job, Cannoneer and Gladiator:
Get when you first enter the Cave.

Unlock Cloister of the Dead:
Get when you earn the Necromancer job.

Unlock Sealed Temple:
Successfully beat the game once.