Demon Rat

The Demon Rat is a hallucination that was seen by Casey Jones when he was sprayed by a fungus spawn in the episode, Fungus Humungous.

History
When Casey reported back to the turtles that he lost April, they went back to look for her. Casey was then sprayed by a fungus spawn, and started seeing rats, the first of which was The Demon Rat. He is seen later in the episode next to a second Demon Rat standing near Casey, while Leo was fighting Fungus Humungous. While the credits are rolling in, you can hear Casey talking to Raphael about the Demon Rat.

Appearence
The Demon Rat appears as dark, shadowy, cloakless version of Splinter with red eyes and no beard.