Supreme Dog is the main antagonist of the Arthur episode, "To Eat or Not to Eat". He runs a company named Rabid Dog produces the Big Boss Bar, which is an addictive candy bar that a slew of characters have been purchasing.
"To Eat or Not to Eat"
The Big Boss bars are advertised across Elwood City and become particularly popular among the students of Lakewood Elementary. Several are bought by Binky Barnes who resells them to his fellow students in the manner of a drug dealer. Buster investigates the cause of the peculiar effects the bars have on the others. The disturbing number of chemicals lead to symptoms such as hyperactivity, "sparkles" emitting from the mouth, and a withdrawal-like phase when those who consume the bars cease to feel its effects.
With his mother Bitzi, Buster arranges an interview with Supreme Dog who attempts to reassure them that the Big Boss bars aren't addictive despite describing the chemicals' addiction-causing effects. Buster tries to get Supreme Dog to eat a Big Boss bar to prove his claim that it isn't addictive. Supreme Dog vocally refuses and flees after Buster pushes him. Bitzi composes an article exposing Supreme Dog's act, which leads to his imprisonment and the shutdown of his company.
Given the Big Boss bars' effect on the students seen in the episode, Supreme Dog is perhaps the darkest villain in Arthur. It also adds a level of horror to his actions, given that many students are shown consuming the bars and experiencing the drug-like addiction and effects. And given how the Big Boss bars are widely distributed, many others might have become addicted as well. Fortunately, they seem to recover by the next episode.
- He is believed to be the only real villain from the Arthur series (unless you consider the Tough Customers as villains in this series), and he's also believed to be the most evil character from the series, because he intentionally distributed addictive candy bars to the children in Elwood City for a profit.