Go to Narshe in the World Of Ruin. Have Mog in your party. Then, go to the mountain where you first got Mog. You will see the frozen Esper from the start of the game. Use Fire attacks on him and defeat him. This is the esper Tritoch that teaches Fire 3, Ice 3, and Bolt 3. Then, go to where the Esper was located and slide down. You will be in a cave. Most of the chests are trapped, and only have a secret path to them -- do not touch them. At the end of the cave, go to the skull statue, and take the Esper. Yeti will attack you. Defeat him, and if you have Mog in your party, he will join.