D20 has creatures set up into creature types already, d20 Pokémon defines some of these types as Pokémon (Aberration, Beast, Dragon, Elemental, Magical Beast, Ooze, Outsider, Plant, Shapeshifter, or Vermin). This way you can translate creatures from other d20 sourcebooks quickly and easily into a Pokémon campaign – simply looking at the creature type you can immediately determine that a Beholder is a Pokémon, but an Elf is not.
Similarly, the Pokémon from the Pokémon world are not type “Pokémon”, they are given an appropriate type from the normal D&D rules. A Bulbasaur is a plant, a Pikachu is a magical beast and so on. This way, a Pokémon can be incorporated into a regular D&D game easily as it follows the standard Dungeons and Dragons rules for monsters.
Pokémasters have their own class, which functions in all ways like a normal player character class in the D&D rules. In a Pokémon centered game virtually everyone will be a Pokémon Trainer, while as in a regular D&D game normally none of the characters will be one. In lighthearted, or mixed Pokémon/D&D games, Pokémaster is another class that players can play – exactly as Fighter or Sorcerer is.
In this way you can add as much Pokémon to Dungeons and Dragons as you want – or you can start with Pokémon and add as much Dungeons and Dragons as you want. In either case, the rules here are meant to be scalable to exactly the mix of serious and silly that you want in your game.
Pokémasters are designed to not over-balance the campaign. As such, the absolute limit on what Pokémon will obey a Pokémaster is set by the Challenge Rating of the Pokémon and the class level of the Pokémaster. Very roughly, a monster is roughly equivalent in power to a single character of a level equal to its CR. A Pokémaster can only control a Pokémon of a CR lower than her own class level – which means that the Pokémaster’s Pokémon should normally be weaker than the other PCs. Furthermore, the Pokémaster herself is somewhat weaker than a Rogue or Wizard of the same level. So a Pokémaster should always be playing two weak characters rather than one strong character.
A Pokémaster has significant leverage over her Pokémon, and can grant big advantages to her Pokémon as well. It is therefore imperative that no PC ever become the Pokémon of another PC. Enforcing that kind of power relationship between two players is not something I think is a good idea – and giving that kind of game mechanical advantage to a player could severely impact game balance. As such there are several constraints put on what creatures count as Pokémon.
The first is that creatures whose “advancement” line says “By character class” are never Pokémon and can’t be caught by Pokéballs. This ensures that all of the “normal” PC races, even some of the powerful ones such as Aasimar are still playable without living in fear of being caught in Pok´eballs.
The second is that Fey aren’t considered Pokémon. There’s nothing inherent about Fey that suggests that Nymphs shouldn’t be Pokémon, it’s just that the spell Reincarnation has a tendency to bring characters back as Dryads and Sprites – and that would make them Pokémon if not for the general prohibition on Fey as Pokémon. It’s rather heavy handed – and if you are playing in a game world where there is no reincarnation (and no Fey PCs), then Fey can be added to the list of what is a Pokémon.
Playing Without Pokémon
The rules given here can just as easily represent summoners of any flavor. The basic concept of the warlock who defeats magical creatures and makes them work for him is as old as Solomon, and massively predates fantasy roleplaying. As such, a Pokémon-free version has been made for those who want to play a more “serious” game with no Pikachu in it, but one that has summoners patterned off of Final Fantasy, the Bible, or any of a number of other literary sources.
(Editor's Note: The Monster Tamer pending inclusion in the Book of Gears is the non-Pokémon version.)