Cockroach

Donatello outfitted a cockroach with a spy camera so the Turtles could infiltrate Kraang HQ and eavesdrop on their evil plans. But the little roach dropped into a vat of Ooze and transformed into a mutant giant. 

Although the Turtles thought they exterminated this menace, an embryonic roach arose from the goo, foreshadowing future problems for our heroes.