A new golfish is attacking my original goldfish...whats wrong?

Its possibly trying to get more territory, for example if you and two other people are forced to share a house with no walls you would eventually fight.. try adding plants or possibly upgrading to give them more space.. you could just have a bully fish, or you could have a male oranda that is trying to breed. There probably isn't anything wrong with your pearl but it may get stressed and succumb to sickness. Its suggested that you quarantine new fish to reduce the chances of transmitting parasites and diseases or other infections that fish can carry. a 10g tank is usually a good size quarantine tank for one fancy gold fish. unlike tropicals goldfish require a minimum two gallons per inch of fish, even petsmart will follow that rule on their display information guides. fancy goldfish can be up to 12 inches, which multiplied by 2 is 24 gallons of water. right now your fish are probably small, so they dont need that much space. You can house three 1 inch goldfish in a ten gallon with good filtration as an example but they grow quickly and will need the water changed more often. Your fish should adjust, one way to help them if its being a bully is to buy a breeder box and have him in it in the same tank, that way they can see eachother but not fight. Or you can purchase a divider and separate using that method, kind of as a "time out" space for your fish until you notice no more aggression.

What size is your tank? The new one could be attacking the old one because there isn't enough space in the tank. Goldfish really should be kept in a 30 gallon tank for just 1, plus 10 gallons for each additional fish, so you should have around a 50 gallon tank for your fish. If they're all really small still then that might not be the problem yet, but as they grow bigger it will be.

It could also have something to do with the levels of your tank being off so I would test your water as soon as you can to see what's going on. For the time being I would separate the new goldfish from the others so he doesn't do any serious damage to the original one.

Yes you should seperate them. Unless your fish are being starved, they shouldnt do that. Some fish just are not able to live with other breeds of fish.

what size is the tank?

have you tested the water for ammonia, nitrite, and nitrate?