For those of you who havent played it here is a VERY brief description for you. Basically, your pet starts out has an egg that you carry with you. Even before birth, it starts with semi random abilities( ex: fire atribute, extra attack, ect). As you fight in the world, the egg slowly gains exp until it finally hatches thus releasing the monster. (Note: As damage is inflicted upon you, the pet also receives damage. Also if the egg/released pet is killed, it takes a special potion to revive it)Once released, you can continue to lvl your monster thus adding more abilities. (Note: as your monster takes damage, it takes potions to heal it.
Now im not saying to copy this system but instead to use it as a refrence for the upcoming system.