You can't.
Granbull is #210 in the national pokedex, and it is a Fairy type Pokemon.
In Pokemon Pearl you can find Tauros in Routes 209 and 210 using the Poke Radar. You can get the Poke Radar after seeing every Pokemon in the Sinnoh Pokedex and getting the National Pokedex.
In the sinnoh pokedex, 210. In the national pokedex, 493
First complete the sinnoh pokedex, 210 Pokemon
u have to see 210 Pokemon to get the national pokedex from rowan
210
giratina is #210 in the Sinnoh Pokédex.
The sinnoh dex has 210 pokemon, and the national dex has 493 pokemon. It has an entire 664 pokemon (I think)
See all 210 of the sinnih dex and talk to oak
210 some are really hard
In D/P you need to have seen 150 Pokemon to get the National Pokedex and for Platinum you need to see 210 pokemon.
Find all 210 regional Pokemon to complete the Sinnoh Pokedex. Then, talk to Professor Rowan in his laboratory.