The pokedex is made based on Pokemon I'd numbers. When you look at their stats in the pc or sumary, there is a number on top. For example, bulbasaur says : dex number : 001. This means, bulbasaur is the first Pokemon in the national pokedex. Some Pokemon may have been created before others, but usually you find them after they created the starters (always goes first). Also, there are several regions in the pokedex. The pokedex is designed to go according to regions : Kanto, Johto, Hoenn, Sinnoh, Unova, and finally Kalos. Each region has their respective amount of Pokemon and some may have more (I believe Unova has the most and Johto has the least). So, here's your answer.
Hope I helped! :)