Altering Cave! How to unlock?

[Cold 09]

OK, so when I go to the altering cave (the six island), only wild Zubat appear.
But, on the net you can find info that says that there are some rare pkmn (like houndour, smeargle, etc.) that can be found only there...
How do I unlock those pkmn, 'cause the only one attacking is the Zubat?
I've also read that the Mistery Gift is got something to do with it...Is it true?
Please reply!!!

PS can I get it with GameShark?

[belmont]

As far as I know you can only unlock the rare pokemon there with a Gameshark or Action Replay.
Anyway all of then can be found in other locations as well.