A Shinigami is a God of Death, and most cultures have, or had, one at some point in time. While we can't know if they truly exist or not, and especially what they look like, we do know that Ryuk looks rather scary even as a plush toy.
Despite all this, he's a beloved character who likes to eat apples. Of course, only Light sees him in the series, but only because it happened to be Light to discover the Death Note that Ryuk dropped out of boredom, into the human world.
Speaking of the infamous Death Note, Ryuk holds a tiny version of it. This product is officially licensed and released by Great Eastern.