Gargoyle (X-Files)

The "Gargoyle" is an unnamed demonic entity that acts as the over-all main villain of the X-Files episode "Grotesque" - though it is never revealed whether the creature is a real demon or simply the product of insanity, making it one of the more enigmatic entities in the series.

History
The Gargoyle was a demonic creature that the serial-killer known as John Mostow became fixated on, the creature invaded his art and had him locked up in various psychiatric wards for several years - eventually the demon, according to Mostow, drove him to murder and mutilate several male victims.

After being arrested and once again locked up for his crimes Mostow continued to state that the demon was responsible for everything and the same style of grisly mutilations and murders continued, this sparked further investigation by Mulder and Scully, who ultimately discover a rogue FBI agent named Bill Patterson who had been working on Mostow's case for three years was behind the new murders.

Patterson is arrested but continues to mirror Mostow in a disturbing fashion, claiming that the demon was behind everything - just like Mostovw it is revealed that Patterson has drawn a gargoyle face on his cell : strongly suggesting both men were now possessed by the creature.