Justin was created by accident when the Turtles and Karai broke into the Kraang's main 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, a beetle, 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 Kraang-droids and the Turtles, but it was finally killed by Leonardo via electrocution.
Despite his apparent death, 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.
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 Michelangelo tried to name it, he gave up and exasperately exclaimed "Let's just call him Justin".