You may have discovered the Triangle Island before the events on the Floating Continent. It is still present in the World of Ruin, and there is a friend you can meet there.
Go to Triangle Island
The Triangle Island is in the northeast part of the world map. Fly there and wander around having random encounters (unequip the Moogle Charm/Molulu's Charm if you are using it) until you fight Zone Eater. Don't attack it, but instead allow it to engulf your entire party. If you want to speed up the process, attack it with any Magic other than Pearl/Holy, and it will counterattack with Engulf. After everyone has been Engulfed, you will be in a cave.
You can leave the cave by standing in the light north of where you entered the cave, but for now, you should go south.
202. Tap Dancer/TapDancr/Amduscias
204. Karkass/Shambling Corpse
205. Wart Puck
After going south from where you first entered the cave, go west and press A to jump to the middle bridge, and allow the green person to touch you. Open the two treasure chests in this room, then go east and step on the switch to lower the rock. Go east and up the stairs.
From there, go west, and jump from bridge to bridge, avoiding the green people. There are two treasure chests in the bridge area that you can open. You have to approach one from the right and the other from the left. There is also a chest in the southwest part of the bridge area.
Go northwest after the bridge area, then go west and south down the stairs.
Use the save point, then go west through the door. Equip the Sprint Shoes, then watch where the ceiling lands before you start running, and watch how long it stays up before falling. When the ceiling goes up, run to the nearest safe spot. There are three treasure chests in this area, so try to open them all.
Save, then go to the next room. Watch where the ceiling lands before you start running, and watch how long it stays up before falling. When the ceiling goes up, run to the nearest safe spot. Try getting all the treasure as you make your way west. At the west side of the falling-ceiling room, go south to the next area. Go south again to reach a large room.
Go south, then press A to jump onto the treasure chest to go across. From there, go north and jump across the chests. Go around the bridge, and then jump from the bridge to the island with a switch on it. Step on the switch, then go east and jump across the bridge, then go south and jump across the chest. Go west again, following the path until you reach a chest in the northwest corner of the area. Open it, then go back to the island with three pieces of bridge sticking out of it. Go north, then follow the loop until you can go west across the chest. From there, go northwest to the door.
Go up and talk to the person above you (default name: Gogo). After Gogo joins you, you can use a Warp Stone/Teleport Stone or the Warp/Teleport spell to leave, or you can just go back the way that you came.
Gogo's main ability is Mimic. If Gogo uses Mimic in battle, Gogo will repeat whatever the last action was. Gogo will not use any MP when mimicking a spell, and Gogo will not use up any items when mimicking a Throw or mimicking using an item. If Mimic is the first action of the battle, Gogo will just attack.
In addition to this, you can assign someone else's ability as Gogo's ability. For example, you can assign Blitz as Gogo's ability, and Gogo will be able to perform any Blitz that Sabin knows. Giving Gogo the Magic ability will allow Gogo to cast any Magic that your party has already learned.
To do this, put Gogo into your party and go into Gogo's Status screen, then add abilities to its list in addition to Mimic.