Mister Chuckle Teeth

Mister Chuckle Teeth is the central antagonist of the X-Files episode "Familiar" of the 2018 revival series. He is one of the show's newest Monsters of the Week and gaining popularity due to its horrific appearance and the tone of the episode being much more in-keeping with the darker elements of the older X-Files over the more comedy-centric stories that followed it.

Biography
Mister Chuckle Teeth started as a harmless child's toy based on a popular child's show (though entirely made up for the show), a small doll that despite its creepy appearance was of no real concern and beloved by its owner, a young boy named Andrew.

However, due to sinister forces at work outside anyone's understanding, Andrew's toy would manifest itself as a horrifying reality, a man-sized version of the doll, which appeared in a forest and lured a curious Andrew into the woods as his mother argued on a phone.

Andrew would continue to follow Mister Chuckle Teeth deeper into the forest unaware that his "friend" was luring him to a shocking and violent death. Being mauled to death by supernatural forces Andrew's death spread fear and discontent in the small town, as well as caused Agents Mulder and Scully to arrive to investigate the mysterious events, leading to a bizarre case of witchcraft and more appearances of Mister Chuckle Teeth (as well as other demonic beings such as a hellhound and a demonic "telletubby" creature) - all connected to a larger threat.

Ultimately, it is revealed that Mister Chuckle Teeth is a demon, alongside all the other strange entities, summoned as part of a curse by a witch to extract revenge on her husband for cheating on her. However, the curse backfired and thus caused the demons to target those she loved rather than her original targets.

The witch set herself ablaze trying to undo the curse and died, seemingly ending Mister Chuckle Teeth in the process but the episode ends with a child's merry-go-round ominously starting to spin by itself, hinting Mister Chuckle Teeth (or the spirit that allowed him to manifest) is still active.

Appearance
Mister Chuckle Teeth is a frightening figure despite his status as a child's mascot, dressed in a tuxedo and having a large head similar to that of a dummy, with strange eyes and a large toothy grin.

He is black and white, save for a large red bow tie, hearkening back to the early days of animation and never speaks, moving in whimsical mannerisms that while seemingly beloved by his audience is unsettling and sinister.

Powers and Abilities
As a demon, Mister Chuckle Teeth is effectively immortal and capable of performing any feat it desires, vastly superior to humans and able to defy all laws of science - it is far from omnipotent, but it must also be summoned to this world (via dark magic) and once summoned seems to pursue whatever task it is set to do, though it does so in a violent and twisted manner that can go against the original purpose of its summoner (a classic case of an evil genie, that grants a user's wishes but may abuse or twist them, thus causing said summoner great trauma).

Trivia

 * The episode is noted to have many similarities and homages to Stephen King, with Mister Chuckle Teeth especially being compared frequently to "It" (this is rather fitting as Stephen King worked on the X-Files before and helped create Chinga).
 * While an iconic monster for the revival era of X-Files some critics have noted that the character design is a little too disturbing for what was meant to be a child's television mascot, stating no sane child's show would star something as obviously scary-looking. However, the character does share some disturbing similarities with Mister Noseybonk from 1980s children's BBC program Jigsaw (this is because he is confirmed by the creators as being directly inspired by the character).
 * His design also resembles Pee-Wee Herman from Pee-Wee's Playhouse.