Bulbasaur was abandoned by the Mayor of Trovitopolis when the mayor was a child because the Bulbasaur would not evolve into a Ivysaur.
To survive in the sewers of Trovitopolis, the Bulbasaur would use its Vinewhip to steal food off people living on the surface. With the strange occurrences happening over the city it eventually led the public to believe that the sewers actually housed a monster.
The mayor responded by sending his personal SWAT team to hunt the monster down to stop it from terrorizing the city. It emerged that the 'monster' was actually a Bulbasaur which the mayor had long abandoned because it would not evolve. Due to his previous cruel acts the city folk refused to re-elect the major and his Bulbasaur was looked after in the nearby Pokémon Center.