It's a Pokemon clone of FireRed/LeafGreen I think that lets you fuse Pokemon in pairs. You can have a Pikachu/Chikorita if you want and each one will evolve independently, so you essentially have 12 pokemon in your party. The fun part is the fusion art that sometimes has some hilarious and cool results. It also has its own adjustments so that it feels less grindy so you can get through both the Johto and Kanto regions easily.

Here's a link to the wiki