Stand By Me:
Gordie: Mickey is a mouse, Donald is a duck, Pluto is a dog. What's Goofy?....
Teddy: He's a dog, he's definitely a dog...
Chris: He can't be a dog he wears a hat and drives a car....
Vern: Yeah, that is weird. What the hell is Goofy?

GCO.com: Well then, it turns out that Goofy isn't so goofy after all, is he? In fact, the contrast between Pluto and Goofy is so startling that the very existence of Pluto proves the point. Can you see them standing next to each other. If you saw them out in your driveway right now, wouldn't you be able to tell which of them wasn't the dog? Can't you see that it would have to be Goofy bent over and petting Pluto, and not the other way around? Now, what dog would do that to another dog?

Disney: A: Goofy is a dog! Good-natured but not that bright, this cartoon character made his first appearance, somewhat disguised, as a member of the audience in "Mickey's Revue" (1932).

[...]

In the newspaper comic strips, this new character was first given the name Dippy Dawg. A 1938 book indicated the first change to Dippy's name, "The Story of Dippy the Goof," and by 1939 the final change was made to Goofy with the release of the cartoon "Goofy and Wilbur."

Goofy was created as a human character, as opposed to Pluto, who was a pet, so he walked upright and had a speaking voice.