After you beat the elite four, get the national dex from professor oak, and go to the new islands. after you do that, the man guarding the cave should be gone.
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You can catch a Houndour in the Altering Cave in Pokemon FireRed.
It's in the lost cave
You can find Murkrow in the Lost Cave in Pokémon FireRed.
icefall cave on island 4
In Icefall Cave, in Four Island