Well, in that video, Goofy and Pluto were a character named Eustace. Eustace bailed Mickey Mouse out on his rent that he owed Pete. Mickey Mouse was so happy that he had a friend that he decided to play Dr. Frankenstein with Eustace. He pretty much turned an anthropomorphic dog or a character that is probably the influence for Goofy into a dog. The dog, Mickey Mouse named the dog Pluto and worried about people finding out that Pluto was Eustace.