Would you sue Savings for making cheaper frozen veggies in a simple packet?Would you sue LG for making mobile phones?
Would you sue Matel for making toy basketballs?
Remember, unless the company is trying to claim that they are making BANDAI tamagotchis they are quite entitled to make virtual pets. Every product has a company that will sell a homebrand version... something that is same as the brand name but cheaper and a little less good quality. Think about it.. you can get expensive shoes and cheap shoes. You can get brand name food products or your can get savings or Woolies brand. There is the game ConnectFour and another company makes LineUpFour which is the same game just named differently and cheaper.
You can't sue a company for making their own version of a product even if it does the same thing. <_< You can only sue if they are claiming to be something they are not... i.e. putting a Bandai logo on their own product.