From Bulbapedia, the community-driven Pokémon encyclopedia.
In each Pokémon game of Generation II, all Pokémon have an unseen index number used to identify them in the game's ROM. Unlike Generation I, there are hardly any glitch Pokémon present, as there are 251 valid Pokémon in Generation II and Pokémon species' index numbers are only stored in one byte (8 bits), allowing only values from 000 to 255 in decimal 00 to FF in hexadecimal). All Pokémon are in National Pokédex order.
List of Pokémon by index number