Bulbapedia list of Pokémon for episode BW142 include Bulbasaur (Ash's) and no other of this pre-Unova pokemon (apart from Charizard).

Ash Start new anime with pikachu and Bulbasaur, bulbasaur eventual evoles in to a vensasaur, gets a mega evolution.

Ash probably leaves it with some random trainer to look after it.