Larvitar was in an egg that Ash Ketchum was given to take to Professor Elm, however in transit of the Egg it hatched into Larvitar. Like any Pokémon just hatched it had the personality of a child, but a tortured child through memories of what happened to its mother. It froze up whenever anyone other than Ash spoke to it or held it, which it eventually managed to get over. Not long after meeting, Ash managed to return it to its mother and stopped the Poachers from ever disturbing them again.
It appears in Be an Arrow! along with tons of other wild Pokemon and seems to have somehow wound up in East Unova, to appear in BW090.