Jade Cocoon is a console-style RPG game for the playstation in which players can catch/tame/collect beasts and use them in battle, however that isn't the best part of the game! the best part of the game is it's merging system which allows players to combine their favorite beasts together.
The merging not only combines skills but it also combines their appearances! you can get a flying creature and merge it with a turtle-like creature to get a flying turtle not only that but you can merge beasts that have already been merged to create a completely new beast. At the end of the game you have an eternal corridor which you keep going through the maps battling/collecting beasts, but it also adds skins to the game that are obtained by beating bosses in the corridor which can be used when you merge beasts.
The beasts/creatures are known as minions. I had ages of fun with this game and loved the gameplay as I could essentially create the minion I wanted, there were well over 100 minions and thousands of possible merging combinations. Sure the graphics are out of date but the gameplay can keep you entertained for months.
There is a sequel, Jade Cocoon 2 for the PS2 but it loses the appearance merge aspect. It does add to the combat as you can have 8 minions out at a time on 4 different sides of your beast amulet (which you can rotate around during battle) that represent elements from the game (Fire/wind/Water/Earth) all of which has it's own advantage (water heals, Fire deals most dmg, earth defends and wind inflicts status effects.)