Hullin 137: Shearing Sheep Only Applies to Sheep

The "first fleece" mishnah applies only to sheep - and the question is how is that known (from the Torah). Along the way, other first-born rules are raised, like the first-born ox doesn't work. Can it be that "shearing" isn't specific to sheep, but removing any animal's hair? And was this a serious inquiry? Also, the minimum number of sheep required for the prescribed shearing -- perhaps 5. Or perhaps according to the value of the fleece. Which comes to a comparable value.