Brick Blair was once a member of Crow Aves's Biker gang before it got disbanded. After Crow became a good guy and an ally of the Turtles, most of Crow's former gang heard of this and decided to reform their ways and become model citizens in his honor, except for Brick. He liked being bad, and went off on his own in a huff, believing Crow had gone soft. This was noticed by Dr Mothman, who offered to mutate him and give him a place in his mutant army, which he gladly accepted. Mutating him with the DNA of a Bobbit Worm, he became a mutant Bobbit Worm and part of Dr Mothman's Navy, hoping to get revenge of Crow for abandoning the life of a bad guy. He was then given the name Scissorlips by Michelangelo due to his slicing jaws that are his greatest weapon.
Art by https://www.deviantart.com/ezio1-3 after asking permission to post his artwork