One interpretation of the Manta Diablo in The Black Pearl is that the Sevillano, a pearl diver, invented or expanded on the mythical creature in order to keep the best diving beds for himself. By sowing fear among the other divers, the Sevillano is able to find the best pearls and keep his reputation. However, this idea is contradicted by the end of the novel, where the protagonist Ramon and the Sevillano both encounter an enormous manta ray:

At daylight the manta still swam behind us. He was no closer than before, swimming along at the same pace as the boat... "I have seen many," said the Sevillano, "but this one is the monster of all. He will measure ten paces across, from fin to fin, and weigh more than two ton... with a mere flip of a fin or a twitch of their tail they can send you into eternity."
(O'Dell, The Black Pearl, Google Books)

Later, the Sevillano harpoons the manta, and is dragged off the boat and drowned. Although it is highly unlikely that the manta in question is, in fact, a vengeful creature of purpose, the manta itself is real; only the legend of the Manta Diablo is invented, and the Sevillano certainly has a lot to do with spreading it. His intentions may be to keep the best pearl beds for himself; the manta is merely a convenient excuse. Interestingly, the largest manta recorded to date weighs about two tons, making this manta easily possible in the real world.