Each time you beat the Gym Leader Castle (including the Final Four and 
your Rival), you will receive one of eight semi-rare to rare Pokémon. 
These Pokémon are awarded at random, so it's likely that you'll need 
to beat the Gym Leader Castle again and again to get the ones you want. 
Note that once you have beaten the Castle, you only need to defeat the 
Final Four and your Rival again if you want to win more Pokémon. 
Here are the Pokémon you can win:
#001 Bulbasaur
#004 Charmander
#007 Squirtle
#106 Hitmonlee
#107 Hitmonchan
#133 Eevee
#138 Omanyte
#140 Kabuto