Negapon
Once you unlock Marvel Lab, enter it. Make sure you have room for at least one more Robopon. You should see a building directly in front of you. Go to the left of that building. Once there you should see a dog. Talk to it. It will tell you to input a password. Enter '9'. The dog should give you Negapon. Negapon is very powerful. It turns into NegponX and Gatspon.