PK3/WC3: Unreleased Altering Cave Pokémon .

In Pokémon FireRed, LeafGreen & Emerald, lies a cave named the Altering Cave.
At first glance, all it has are wild Zubat, which makes me people ask "why the heck does this cave even exist to begin with".
As it turns out, there is more to this cave than it meets the eye.

This cave was programmed to make use of special Mystery Gifts, that would allow the wild Pokémon species present to be altered. (The species present changes every time one talks to the deliveryman)
Sadly, this game feature never had an event to make use of it.

Following it's initial release in Pokémon FireRed & LeafGreen, all the Pokémon that could have been caught from here,
could also be encountered in the extended Safari Zone of Pokémon Emerald.
(with the exception of Smeargle: it can be caught in Artisan cave in Battle Frontier)
Despite these Pokémon all being available in Pokémon Emerald, it's not known why this cave was also included for it.

This page hosts the Pokémon that can be encountered in an altered Altering Cave, as well as other means to get them to appear.
 

New update!
This page also holds a custom wonder card, that can be used to cycle through the encounters of Altering Cave, whenever you talk to the deliveryman!
Take note: if you toss/remove the wonder card, the encounters in Altering Cave will be stuck at the last species encountered.
Also note: FireRed/LeafGreen & Emerald have their own wonder cards. FRLG cannot use Emerald wonder card and vice versa.

The encounters that get cycled are (and it wraps back to the start):
Zubat, Mareep, Pineco, Houndour, Teddiursa, Aipom, Shuckle, Stantler, Smeargle.

International Wonder Card (ENG FRE GER ITA SPA) Card is English language, but works on the languages listed.
Japanese Wonder Card

(Hint: You'll need the WC3TOOL or WC3Plugin to inject wonder cards into your save.)

Also, PKHeX usage:
Pokémon FireRed and LeafGreen: Event constant 36
Pokémon Emerald: Event constant 62

Cycling through 0 to 8 in the above given event constant, results in the rotation of the species that appears.

Also, for cheat code usage:
Pokémon FireRed and LeafGreen:

Pokémon Emerald Codes:

Credits to the Pokémon: @HaxAras
Credits to codes: @BlackShark

Original Post: