This is actually something i copied off another person; Darthknuxward from tmnt2012 wiki.
After the events of Metalhead Rewired in which several captured mutants were freed form the Kraang Newtrailizer, WingNut, Pete, Dr. Rockwell, and Slash joined up to figure out the cities landscape so they could survive as well as to protect eachother.During this time WingNut and Rockwell began to learn how to speak again. Meanwhile Leatherhead was freed from imprisonment from the Kraang Elite in Dimension X. After escaping to NYC he meet up with the group and began to attempt to contact the Turtles. At the same time Ice Cream Kitty ventured to the home of this mutant groupa and Leatherhead and the others followed her back to the Sewer Lair. Finding April, Splinter, Metalhead, Irma, Kurtzman and Casey who informed them that the Turtles were of to fight the Mutant collective. Going to the base of the collective, they helped the Turtles defeat them and formed the Mighty Mutanimals!
The Turtles greatest allies!