From Bulbapedia, the community-driven Pokémon encyclopedia.
In each Pokémon game of Generation III, all Pokémon have an unseen index number used to identify them in the game's ROM. The majority of them are in their National Pokédex order, in fact, all Pokémon from 001 to 251, inclusive, are in the same position as they are in the National Pokédex.
It is important to note that all the Unown data is stored in the index number 201, meaning that index numbers 413 - 439 (Unowns B - ?) are empty spaces, which are occupied only by sprites. Unown letters are determined by their personality values and they use the different sprites (index numbers 413 - 439) depending on the personality values. However, in Pokémon XD: Gale of Darkness, the Unown letters have been moved to the index numbers 415 - 441, to make room for Bonsly and Munchlax.
List of Pokémon by index number