Dr. Toadly
He may look like a normal Toad, but nothing could be further from the truth. This conniving villain is the brain child of seiromem, and he's been a thorn in everyone's side since his introduction in Super RMN Bros. 3. He appeared as the boss of three different stages in that game, and appeared in both Castles ~ Masterpiece Set and another personal project of seiromem's. The mad doctor seems to find his way into anything his creator takes part of, Mario or otherwise.

Being prone to long-winded speeches and having truly devious lairs and factories, he's enough to get on anybody's bad side. Fortunately, he's plagued by the tendency of many cartoon geniuses to thwart himself through his own oversight, or just incompetence in general.

Fun fact: there is an unrelated character named Dr. Toadley who appears in Bowser's Inside Story.