How To Get Larvitar in Pokémon FireRed/LeafGreen Version
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Note: You must become Pokémon League Champion before finding Larvitar.
You must catch at least 60 Pokémon in order for Professor Oak to upgrade your Pokédex to the National Pokédex.
You must complete the Team Rocket Side Quest on One Island at Mt. Ember to retrieve the Ruby for Celio in exchange for the Rainbow-Pass.
Larvitar has a 5% encounter rate at Sevault Canyon, be patient enough and you'll find it. It will be at Level 15 or 20.