[Cerberus87]

Pokédex position doesn't mean anything. Chimecho was the last Pokémon in the National Dex in 3rd gen but it obviously wasn't a legendary (it was damn rare though).

They're harder to catch than most Pokémon in their level, but not really hard. Raising them is harder, though. My first Tyranitar was a real accomplishment!

Larvitar can only be found in mountainous places. The only really large mountain in GSC is Mt. Silver (Mt. Mortar is too early into the game), therefore, it can only be found there.