Colonists

The Colonists are the main villains of the X-Files mythos and are inspired by the alleged real-life entities commonly known as Greys - unlike the real-world Greys however the Colonists have been confirmed as hostile in the X-Files canon from the start, save for a very few special individuals who are not affiliated with their species plan on world conquest.

The Colonists abduct humans and subject them to horrific experimentation while also entering into secret deals with government agencies to provide humanity with special technologies and information in exchange for co-operation with their plan to colonize Earth: however the government agencies are fully aware of the fact that the Colonists are highly skilled liars and are most likely using them, when their plan comes to full fruition humanity may well be treated as cattle at best and vermin at worst.

The Colonists are rarely encountered face-to-face, as is their otherworldly threat, though when they are it is almost always a terrifying experience for their victims and although they tend to return people after abduction some people have gone missing entirely due to Colonist activity (as was supposedly the case of Fox Mulder's sister).

Much like their real-world influences the public's view on the Colonists tend to fall into three camps: the majority of the world don't believe they exist, a small number believe they are benevolent or even godlike and a sizable minority believe they are malevolent and deceiving.

Hoax?

Some episodes in The X-Files have implied the colonists were just a hoax. In the revival event series in 2016, it is believed that aliens had little human contact and their apparent planned colonization of earth (which didn't seem to happen) was just a smoke-screen for a bigger conspiracy. A flashback to the Roswell Incident in 1947 shows agents gunning down and killing an alien who crawled away from it's spaceship injured. Their technology was then salvaged and used to create the hoaxes. A similar idea came up in season five when Mulder was convinced by a government operative that the aliens were hoaxes used to mislead him, however this appeared to be the contrary.

(these events were likely added for later lore to explain why the invasion did not take place as predicted and prolong the show's length - as a general rule aliens are seen as real in the X-Files mythos)