he is a dog
No its mickey mouse's dog :d !!!!
The most probable reason is because Walt Disney made goofy as a lovable best friend to mickey, and in which could be used as a main character of movies to make money which then could be trailed down...
It is television. These things don't have to be explained.
Goofy is also a dog, though an anthropomorphic one. He is Mickey's best friend.
Yes, Goofy is a dog. And talking dog, that is!