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Continuing nonsensically, we go to River.
Bulbasaur is here. Right at the beginning of the level.
Now, I haven't figured out what exactly to do with these little guys, but similar to the Charmander and Squirtle, you can lead the little guys with apples.
Charmander calls out extra buddies and will follow the apples you throw as long as the throws are not too far away. Squirtle and Bulbasaur also follow your apples, at different places.
Leading other Pokemon in the game with apples, like Pikachu in the beginning level (Beach) or Slowpoke in the River level, result in better pictures or evolutions.
Now, do you have to lead the Bulbasaur somewhere? Do you have to lead all 3 somewhere? Where must you lead them? Do you knock them all out instead? These are questions that I believe would be best answered by the pokemon community.
(Trying all these possibilites myself is rather time consuming.)
Now on to further proof of hidden Pokemon, and what other Pokemon may be found with some digging.