Justin is a mixture of various species. He has a pink octopus body with approximately eight tentacles, he has a snake-like neck with a lined mouth filled with teeth and fangs, and a large blue isopod shell on his head. Behind his neck are yellow, green and orange piscine fins that slightly resemble a cobra hood, two purple jellyfish that hold his eyes lined with stalks, and has orange pointed ears like those of a cat. He also has a long, forked tongue like that of a cobra.
Justin was created by accident when the turtles and Karai broke into a Kraang lab called the Worldwide Genome Project. The Kraang had collected DNA samples from every animal on Earth, and when Karai pressed a button on a giant tank of mutagen, the samples poured into it. The creature that resulted was a bizarre combination of DNA from various animals including (examples of the DNA samples seen are a jellyfish, an isopod, an octopus, a cobra, and a house cat). Michelangelo settled on calling it "Justin" when he couldn't think of a proper nickname for the confusing creature.
Justin went on a rampage almost as soon as it was born. It made short work of the Kraangdroids and the turtles, but it was finally defeated by Leonardo via electrocution, after which it broke out of the lab and escaped.
Justin later appeared in Metalhead Rewired as one of the detained mutants in the Kraang prison. He appears in the background along with the Spy-Roach during the scene when Dr. Rockwell beats up one of the Kraangdroids. He fell several feet after a Kraangdroid shot him, but he survived (off-screen) and escaped.
- When trying to name the super mutant, writer Kenny Byerly decided to call him "Justin" after one of the production assistants, Justin Nix.  This may have been mirrored in the episode itself, for when Mikey tried to name it, he gave up and exasperatedly exclaimed "Let's just call him Justin".