Back in 1990, subsistence hunters in Greenland gave a skull to scientists. Its origin, they explained, was a strange creature, evenly grey, with flippers like a beluga but a tail like a narwhal. The specimen was larger than either a beluga or a narwhal’s skull, but seemed to contain the proportions of both.
Source: Gizmodo – Odd Skull in Museum Finally Confirmed as a Narhwal-Beluga Hybrid