You can catch a grass type Pokemon but catching mankey is the easiest because it appears a lot especially to the east of Pewter (where Brock is).
Mankey has the advantage of being a fighting Pokemon which is super-effective against Brock's Pokemon.
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NO he cannot. only Your lead Pokemon can follow you.
After you've beaten Brock go beat the other six leaders then you can fight Blue
to pewter city
Where you beat Brock. An old man is standing in a garden.
Raise your charmander to lvl. 13 and it will learn metal claw. Use Metal Claw to defeat Brock since it is super-effective against rock-type Pokemon.