If you want to fill your pokedex, you must trade between Ruby and Emerald to gain the Pokemon that are not in Sapphire.
You have to catch 151 pokemon to complete the pokedex.
You get the Pokedex after choosing your starter Pokemon in Prof. Birch's lab.
You have see to many different pokemons. Then you will fill it (you have to every Pokemon to get national pokedex).
catch all Pokemon
It is grimer
The original Pokedex is given to the player by Professor Birch. After the Pokemon Ruby trades with LeafGreen or FireRed the National Pokedex will be unlocked.
beat the hoenn pokedex and go to prof.birch
you need 200 Pokemon to fill the whole pokedex
http://pokemondb.net/pokedex/national
www.serebii.net/pokedex-rs/
the 151st Pokemon in ruby's regional pokedex is chimecho. you can obtain one by leveling up chingling at night when it's happiness is high enough