The first time we see Goofy is in the 1932 film "Mickey's Revue." (He was called Dippy Dawg then.) Additionally, his floppy ears and pointed nose are distinctly canine.
The main reason people assume Goofy is a cow is because he has an on-again, off-again relationship with Clarabelle Cow. Additionally, his rounded teeth have a bovine quality.
To get the answer once and for all, we need to go to the source. Art Babbitt is Goofy's original creator and Bill Farmer is his present-day voice. What do they say?
When comparing Goofy with Pluto, Babbit said "One is a dog. The other, human." Farmer said "Goofy is not a dog" but “seems to be in the canine family”. Farmer dubbed him Canis Goofus.
The best way to describe Goofy is that he is an anthropomorphic dog — an animal who exhibits human characteristics.