What is the earliest point I could catch or obtain a larvesta?
Relic Castle, B1F, 04% chance, according to the documentation.
EDIT: By the way, not only the earliest, but the only one besides the event Volcarona being bred.
EDIT again: The boy before Cheren's Gym, inside Aspertia's Trainer School, still says that the only fighting type around is Riolu, when Mankey is also obtainable.
EDIT once more: Captured a Croagunk, real cute and all, though its moveset wasn't really compatible with the list in documentation. Even though it was at lvl 12, its moves were: Astonish, Mud-Slap, Poison Sting and Taunt, in this order. And then, I leveled it up to 15, when it learned Pursuit. So, it's still with its old movepool.
EDITing spree: Riolu learned Feint instead of Force Palm at level 11. And Force Palm instead of Feint at level 15.