I don't know.. but it worked for me: first you'll have to be a girl character and then use
bayleef as a name then when you are in the truck save! as in don't move! after that: of course u will go to ur house then u'll go to brendan's house and go across his bed on the right one then press A on the pokeball then go prof. birch then you will go to prof. birch in the grass:


:( prof. birch :< zigzagoon
ledge
bag
-----go here then prof. birch says don't leave me here! then press the bag from below. there it will be treecko, cyndaquil, squirtle. P.S. it will not always happen.