Nope, it is impossible to get a Larvitar that knows Megahorn. The only moves that Larvitar can learn through breeding are Outrage, Stomp, Focus Energy, AncientPower, and Pursuit. In G/S/C, the only Pokémon that can learn Megahorn are Heracross and Smeargle. There are more Pokémon that learn Megahorn in the Ruby/Sapphire and Fire Red/Leaf Green games. However, even in those games, Tyranitar can't learn Megahorn.