Because it is often seen lurking in landfills, some people claim GRIMER is actually the result of toxic mutation from trash. It has been proven that they don't appear in cleaner areas nearly as often.
GRIMER's body of sludge is actually the result of some toxic mutation from trash. The more it gathers around trash the more smelly its body becomes.
GRIMER is thought to be the result of high levels of pollution in industrial areas. This is because it is most commonly seen around Collosus City.