|Megatox did not originate from fanon, and more information can be found on their respective wiki(s). You may read more about it on the following wiki(s):|
Megatox is an artificial character from the Sonic series. It is the aftermath of Dr. Eggman flooding Chemical Plant Zone in Sonic the Hedgehog 2 in an attempt to drown Sonic, a move performed unknowingly of his assistant being downstairs; after Eggman and Sonic have discommunicated themselves from the plant, Megatox 'blows up' and becomes one with the Mega Mack. Megatox makes a couple of later appearances in Sonic the Comic, but by and large he isn't seen since.
In Sonic Heroes 2, he is proven to be dormant. In Team Badnik, immediately before Team Badnik hit Launch Base Zone, Team Eggman ascend in the Egg Carrier; at this point Team Badnik have just entered Chemical Plant, and Megatox blames Team Badnik for the racket, and a boss fight begins. After this, Megatox realises that it was Team Eggman who was responsible, and the team and Megatox make a pact.