In B/W, you are given a Larvesta Egg if you enter a house on Route 18. This is pretty much after you've already beaten the game, but did you hatch the egg and use the Larvesta on your team at all? Btw, this is the only way to get a Larvesta in Black and White without breeding a Volcarona for one.
You can explore Route 18 as soon as you get Surf if you so choose, so you can get the Egg as early as around the time of Clay's Gym. I myself went and picked it up (as well as Waterfall nearby, opening up even more areas to explore) but didn't actually hatch it. It's very strong and narratively quite important to Unova lore, so I get why, but it just takes so long to hatch, and I already had a Fire-type on my team.
I did get the Egg after obtaining Surf in BW but I didn't use Larvesta on my team after hatching it since it evolves so late. However I was lucky to hatch a Modest Larvesta so later when I played postgame of BW2 I traded it over, EV trained it and used it on my PWT team. That Volcarona from BW Egg (together with Gyarados and Amoonguss) helped me beat BW2's PWT tournament (including the Champions tournament) for the first time ever.
god that thing takes so long to hatch AND evolve. it's such a great pokemon though it's worth if it you don't want to use any of the other unova fire types, i suppose. i've never used that particular larvesta in my party, but now a rattata gotta consider it... lol