Answer:
It is a major controversy, but no one really knows, they just guess. Here are some arguments for and against:
Walt Disney wanted the animals to NOT be transspecial dating. That is why Mickey and Minny MOUSE date, as do the ducks. Goofy dates Clarabell, so why wouldn't he be a cow?
If Goofy was a dog, wouldn't he bark at least once? Pluto barks all the time, but Goofy never does.
Pluto has paws, but Goofy has three fingers on his hand. Doesn't that mean that he's a cow, since cows have three "toes".
A dog, but i really never understood why Pluto walks on all four paws and goofy doesn't and why goofy drives a car and Pluto doesn't.