"I, Monster"
This story takes place before Teenage Mutant Ninja Turtles (1984) #19.
First appearance of Rat King.
Collected in Collected Tales of the Teenage Mutant Ninja Turtles Vol 1, Tales of the Teenage Mutant Ninja Turtles (2007) Vol 1 and Tales of the Teenage Mutant Ninja Turtles (2012) Vol 1.