Only Zubat can be found in it. It would have been possible for the Altering Cave's wild Pokémon to be altered by using Mystery Gift at a Wonder Spot (hence the cave's name), but this data was never distributed.
In Pokémon Emerald, Altering Cave is found on Route 103, on the east bank of the inlet that passes through the center of the route. The entrance of the cave is only open after the player enters the Hall of Fame.
Pokémon FireRed and LeafGreen use standard wild held items for the Pokémon in Altering Cave; Pokémon Emerald uses a special set of held items exclusive to the event, which were programmed to have a 5% chance of occurring.
A colored background means that the Pokémon can be found in this location in the specified game. A white background with a colored letter means that the Pokémon cannot be found here.
Had a distribution been held to change the wild Pokémon in Altering Cave, this would have been the only way to obtain the Berry Juice in Generation III, as wild Shuckle are programmed to always be holding it in Pokémon FireRed and LeafGreen, and Shuckle and Aipom would have held it 5% of the time (in Altering Cave only) in Pokémon Emerald. Instead, the item is entirely unobtainable in Generation III.
It would also have been the only way to legitimately obtain Mareep that are under level 5 (Down to level 3) in Generation III, since all Pokémon in that generation hatch at level 5, and no other obtainable Mareep within Generation III are under level 5.
