Klunk
Klunk is Michelangelo's pet cat. He first appeared in the Michelangelo microseries, and was hit by a car and died in the Tales of the TMNT vol. 2 issue 9. Shortly after, the Turtles discovered that Klunk had mated and had kittens with an alley cat.
Klunk also appears in a few episodes of the 2003 cartoon, starting with The Christmas Aliens. Based on the revamped character designs in the Back to the Sewer season, this version of Klunk appears to be female, whereas the Klunk from the Mirage comic was confirmed to be male.
Read more about this topic: Michelangelo (Teenage Mutant Ninja Turtles)