Megatox is an original villain from Sonic the Comic, which was the United Kingdom's official Sonic magazine - in contrast to Archie comics.
Megatox is a monster made out of the pink liquid known as Mega Mack (the infamous pink "water" found in Chemical Plant Zone in Sonic the Hedgehog 2, also referenced in Sonic Generations).
Originally Megatox was a scientist working alongside Dr. Robotnik in the creation of Mega Mack as a means of disposing of Sonic once and for all - yet like so many experiments in fiction his was doomed to failure and through some miracle (or curse) the scientist survived what should of been a lethal dosing of the toxic substance, his body merging with the stuff and transforming him into Megatox.
He would go on to attack Sonic several times, at one point trying to drown him in his sludgy body, an agonizing way to die. He also occasionally worked with other villains in the comic for his own selfish needs of killing.