Dolgat in Diamond and Pearl
 

Dolgat is found in Magikarp form at exactly 3:25 AM by fishing off of the Secret Pier. The Secret Pier is very hard to get to. First, you and a friend have to be registered on each other's pal pads and battle online. Then, you both have to turn your DSes off when the first Pokemon is thrown into play. When you turn your DS back on, you will be on the Secret Pier. There will be 2 fishermen there. Battle them and then you will be able to fish off the pier. The first Pokemon that appears will be a Seaking. Knock it out, and then the Dolgat Magikarp will appear. Catch it and rename it Dolgat.