Outside the Weather Institute on Terajuma, if you show the person the 3 weather TMs (Hail, Sunny Day, Rain Dance) you get a Castform Crest, which activates a weather move upon entry and confers an additional ability based on weather (rain dish, solar power, and I can't determine the one for hail/sandstorm).
I read someone else mention crests for all John starters and Ledian, but I don't know how to get those and am especially curious on how they could make the latter remotely useful.
Zangoose/Ariados are in the underwater temple in route 6.
