Who was that really weird trainer that stole a Larvitar?

The trainer you're referring to is probably Team Rocket's Jessie, who, along with her partner James, attempted to steal a Larvitar in the Pokémon anime. This incident occurs in the "The Larvitar Case" episode of the Johto Journeys series. Jessie’s motives were often driven by her desire to capture rare Pokémon, which made her actions seem particularly eccentric and memorable.