Geists appear in groups together, with Ghouls, or with Specters, Images, and Wraiths, and are found in the beginning to the middle of the game.
A whole pack of geists can be taken down by one shot of FIR2, but if they catch you off guard and stun you you could be in for a slow, painful death.
Geists are called Ghasts in the Japanese version and FF Origins. Ghasts come from Dungeons & Dragons. They are stronger, smellier versions of ghouls. The old English word ghast means to cause fright or shock. It's the base for the word ghastly, which means horrible, frightful, or ghost-like. Ghasts can also be found in FF2 and the English version of FF4. The name geist is an abreviation for poltergeist, which is an invisible, mischievous spirit known to throw things or make noises. They were made famous by the movie, Poltergeist, and can also be found in Dragon Warrior.