Unofficial custom Pokémon games can be designed and played online. Pokémon users and fans can design and build custom ROM hacks Pokémon games and play them for free, either online or on recycled Game Boy Advance cartridges. Examples of these custom games include Pokémon Advanced Adventure and Pokémon Prism.
Pokémon Advanced Adventure centers around the player as he battles his rival Gary, a dangerous psychotic that rules the Pokémon world. The player battles tyrant Pokémon on each game level. To beat the game, the player must kill or capture each tyrant Pokémon.
Pokémon Prism is a modified version of Pokémon Gold, and features modified Pokémon from the first four series generations in an 8-bit aesthetic.