Chocob - Damage 1 enemy. Small chance to get "Fat Chocobo" instead, which damages all enemies.
Sylph - Steal 1 enemy's HP and give it to your party.
Remora - Latch on to 1 enemy.
Shiva - Damage all enemies with ice magic.
Shiva is found behind the largest waterfall in Worus Castle.
Ramuh - Damage all enemies with bolt magic.
Ramuh is found in the forest east of Istory. Wander around randomly until he appears.
Ifrit - Damage all enemies with fire magic.
Ifrit is found in the Ancient Library.
Titan - Earthquake damages all non-flying enemies.
Titan is found in the Karnak Meteorite.
Golem - A wall protects your party from physical attacks. It can withstand several thousand HP of damage.
Golem is found in the Valley of the Hiryuu (World 2). You will meet him randomly in battle. If you attack the other monsters (and not him) for a couple of battles you will get him.
Shoat - Destroys one enemy. Doesn't always work.
After you get the sub in World 2 you'll see a white dot on north-western part of the world map.Go to that place and go through the cave. When you exit the cave, wander around and you will encounter Shoat.
Garnet/Crbnkl - Casts "Wall" on your party.
Found about half way through Exdeath's castle in World 2. You have to make it to the upper-most skull tile in the diamond-shaped area with the pitfalls located right before the final save point.
Syldra - Damages all enemies with bolt magic.
Syldra is found at the pirate base after you get the airship in World 3.
Odin - Destroys all enemies. Doesn't always work.
Odin is found behind the locked door in the basement of Bal. To get there you must climb the wall at dead end in Dungeon of Jacole in World 3.
Phoenix - Damage all enemies with fire magic and revive one character.
Phoenix is found in the Phoenix Tower, the one in the middle of the desert in World 3. To get there you must get the Black Chocobo in the Town of Mirage. Gather info about the Town of Mirage in Crescent.
Leviathan - Damage all enemies with water magic.
Leviathan is the boss of Istory Falls. After obtaining the tablet, don't use Exit. First walk to the outer ring of land and fight Leviathan.
Bahamut - Damage all enemies with non-elemental attack.
Found at the top of the North Mountain in World 3. It is in the desert like Phoenix, except up and to the right of the patch of trees you land in.