When Magikarp evolved into Gyarados, he did it through pure effort; working hard each day until he was powerful enough to beat the odds and leap over waterfalls, earning him the right to fly right over whatever he wished. For a while the newly evolved Gyarados was amazed by his brand new abilities. He felt unstoppable. But as time went on he became greedy. His strength wasn't enough. He wished to evolve again and gain even more power. He trained and trained, but no matter what he gained, it paled in comparison to the rush of power he felt the time he evolved. Hard work and effort just didn't seem to be enough. So he sought a way to get stronger using outside sources. And eventually he came across the power of Gyaradosite; a stone that would grant him the ability to evolve even further than his current form would normally allow. But the stone manipulated the darkness within his heart. The pure, hard working sea creature that he used to be was no more. There was only the physical manifestation of a dark and greedy lust for infinite power. And so as the darkness took over his body, he lost the leaping and flying abilities his old self had worked so hard to earn in order to make way for the new malevolent powers the stone was to bring him.

And that, /Vp/, is why its a Dark Type.