Fungus Humungous - 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. Later in the same episode Casey is surrounded by two Demon Rats, while Leo was fighting Fungus Humungous. While the credits are rolling in, you can hear Casey talking to Raphael about the Demon Rat.
The Demon Rats are a dark version of Splinter without his robe and beard, and both have the same white markings on their noses.