Zubat.
Nothing but Zubat.
You can't do anything with altering cave unless you have an E Reader.
Nothing.
altering cave,
Six island its in the north.
North in six island.
In there you can only find zubat but the reason why its called the altering cave is because you can change what appears inside using a E reader device and E reader cards.
You can catch a Houndour in the Altering Cave in Pokemon FireRed.
Probably.
It is a mysterious cave filled with Zubat. If you did something with the Mystery Gift and a Wonder Spot, there would be 8 different Pokemon found there, but Nintendo hasn't yet done anything with it. In summary, it's useless (unless you need a Zubat).
Well normally you can only find zubat in there but the reason why its called altering cave is that you can change what you find in there by using an E reader machine and E reader cards.
Altering Cave in Pokémon FireRed on Six Island is a special cave that typically only has wild Zubat within it however during a Nintendo-sponsored event, Nintendo would be able to send you a download via the Mystery Gift feature that would change the wild Pokémon that appear in the Altering Cave. Pokémon that are capable of appearing in the Altering Cave via the Mystery Gift download are Mareep, Aipom, Shuckle, Pineco, Smeargle, Stantler, Houndour and Teddiursa however there is no way to change the Pokémon appearing in the Altering Caves due to Nintendo no longer releasing distributions for Pokémon FireRed.