Some TMNT stuff really isn't for little kids.
In the IDW continuity, Mutagen is a mucilaginous substance that has the effect of causing any creature exposed to it to increase in size, strength, and sometimes in intelligence. It is responsible for mutating many of the characters in the series, particularly the four Ninja Turtles.
Mutagen is derived from a substance typically referred to as ooze, which accounts for its effect on those exposed to it. The most prevalent source of mutagen was from Stockgen, which was contracted by General Krang to generate a super-soldier serum that would allow him to have near-invincible troops.
However, the mutagen was accidentally dropped during a raid on Stockgen by Foot Clan ninjas, causing the mutation of the Turtles, their father Splinter, and the cat Old Hob. The subsequent theft of mutagen from Stockgen allowed for the creation of many other mutants, both by other mutants and by the Foot Clan. It is also apparently available to the Null Group, though it is not clear how they came by it.
Most of the mutants produced by mutagen were originally animals, including Alopex, Koya, Bludgeon, Slash, Herman, Pete, and Mondo Gecko. The effects of the mutagen typically involve the animal becoming more humanoid in appearance, sometimes growing limbs and teeth, adapting to their environment and becoming bipedal rather than quadrupedal. They also often become more intelligent, although a certain psychotropic compound is required to guarantee this, as intelligence is not a certainty.
As the mutagen was originally meant as a super-soldier serum, it has a very different effect on humans. In humans, the mutagen gives the subjects increased strength, muscle and overall inducing a state of peak physical condition.
Additionally, mutagen can be used to mutate humans such as Bebop and Rocksteady, if animal D.N.A. is introduced into their system. In this case, the intelligence of the subject will be unaffected, but their size, strength and physical appearance will all change drastically.