Dr. Sigmund Fruitcake
From the Super Mario Wiki
In the live-action segment "Bonkers from Yonkers", Dr. Sigmund Fruitcake is called to Mario Brothers Plumbing by Luigi to help Mario who, after receiving a head injury, believed himself to be a chicken. Arriving quickly at Mario Brothers Plumbing after being called, Dr. Fruitcake accidentally mistakes Luigi for Mario and says that, aside from its ugliness and hairyness, Luigi's head was fine.
Luigi, somewhat insulted, directs Dr. Fruitcake to Mario, who is the one with the head problem; being told that Mario believes he is a chicken, Dr. Fruitcake tells Luigi that Mario should be fine, but promptly changes this opinion after seeing Mario making obnoxious clucking noises.
Later, after having performed some treatment on Mario, Dr. Fruitcake is called by Luigi again, who says that Dr. Fruitcake's treatment to make Mario believe he wasn't a chicken anymore only partly worked, as now Mario was acting like a chicken and a monkey.
Returning to Mario Brothers Plumbing, Dr. Fruitcake, after inspecting Mario and telling Luigi several bad jokes, begins trying to use "thought-transfer therapy" to cure Mario. After yelling "I am not a chicken!" and "I am not a monkey!" at Mario several times, Dr. Fruitcake manages to return Mario to normal, only to discover that Luigi now believes he is a chicken. Dr. Fruitcake, nervous, decides to simply leave the apartment and not try to change Luigi back to normal.