Perhaps closer to a "phenomenon" than a "monster," the Psychodrome is an ordinary electronic display, usually a television, that has bonded to the nearest mind upon receiving a mysterious extra-dimensional signal. Through this curious connection, the device becomes dimly self-aware and functions for its new master as a psychic feeding apparatus, constantly receiving and metabolizing the thoughts of other creatures through a bizarre array of organic antennae.
The Psychodrome seeks to prolong its abstract existence by any means necessary, becoming fiercely protective of its master. Capable of broadcasting pure psychic chaos, it can warp the perceptions of others and even cause permanent brain damage or memory loss.
The force by which these entities are created has proven difficult to study, but may represent the heavily deteriorated echoes of ancient mental warfare. Psychic activity is believed to ripple through the dimensional continuum indefinitely, and on rare occasions, may be "bounced back" to conventional reality in a heavily distorted form. What these brainwaves are actually "bouncing" off from is not known; more than likely for the better.
Another old one that was nearly scrapped, and one of the toughest yet to write description for. I'm still not sure I got the idea across quite how I was thinking, even with the formidable power of vague technobabble.