Doctor Whosit

Dr. Whosit is a television show and a parody of the real-life show Doctor Who.

Facts
Not much is known about the show, but it seems to be very popular as evidenced by most, if not all, of Arthur's classmates knowing of it and everybody in the ice cream shop referencing it.

It also seems to have some elements of horror as Mr. and Mrs. Read seemed unnerved while watching it and disallow D.W. and, eventually, Arthur from watching it. Part of the horror seems to involve something called "cosmic goo".

Unlike the actual Doctor Who series, it appears that Dr. Whosit is the actual character's name and that, like the Doctor from Doctor Who, he can time travel, as evidenced by Buster asking why Dr. Whosit didn't go back in time.

There is also a separate character with "Egad!" as a catchphrase and creatures called "cyber manatees". The protagonist also occasionally does something known as flibbering, but it's never revealed what exactly that is.