Greys

Greys are a hypothetical alien species often associated with alien adbuction stories, ufology and New Age beliefs - although such beings would normally not be considered villainous the Greys do deserve some notice for the fact that many encounters with these supposed creatures are said to be hostile, though some are more peaceful in nature.

Greys are generally believed to be untrustworthy and dangerous with stories involving attempts to crossbreed with humanity or working in alliance with Reptoids, another supposed alien species that is almost universally described as malevolent in regards to humanity - ever since reports of alien abduction became known to the public the idea of the Greys became an instant hit and they continue to fascinate people, though the debate still rages on whether or not they exist and if they are good, evil or beyond classification (the latter seems the most probable if they did exist).

More importantly in regards to villains the Grey have inspired a host of often hostile antagonists for movies, television series, comics and books as well as videogames and other media - though they often have different names and are depicted slightly different in each story.

Famous examples of characters based heavily on the Greys (who are antagonists) would be Crypto from Destroy All Humans and the Colonisers from the X-Files.

Media That Has Greys As Major Antagonists
The following media have Greys (or very similiar species) acting as the main antagonists:

The X-Files

Destroy All Humans

Fire In The Sky