The EPA Scientist appeared (seemingly from out of nowhere) right after the Simpsons discovered that the Screamapillar had taken up residence in their new Koi Pond. Citing the "Reversal of Freedom Act of 1994," he informed them that because the Screamapillar was an endangered species and had chosen to live in their backyard, they were responsible for its safety and well-being. He gave them a booklet with instructions on how to care for the Screamapillar, and then left.
The EPA Scientist returned when Homer believed he had accidentally killed the Screamapillar and decided to bury it on the sly -- partly to avoid trouble with the EPA, but mainly just to be rid of the Screamapillar. This time, the EPA turned out in force with helicopters. The Screamapillar was okay, but Homer was arrested on the charges of "attempted insecticide" and "aggravated buggery", for having tried to duck out of being responsible for the Screamapillar's care. When Homer was tried on the charges, he was found guilty and sentenced to 200 hours of community service.
The EPA Scientist's name isn't specified. He mentions having had a wife, who left him. His appearance and mannerisms are most likely based on Willam Atherton, who played the secondary antagonist Walter Peck in the film Ghostbusters. In the film, an EPA bureaucrat's enforcement of mindless regulations shut down the Ghostbusters business and demand the NYPD open the Ghostbusters' containment unit, allowing the dangerous phantoms to wreak havoc on New York City.