Rashi seems to have had the same problem as you. This is what he comments on 'And not the milk of a male':

'That it (the male animal) has a bit of milk from its udders, and it is speaking when it changed its nature, and has udders'.

So the Torah is speaking about a freak male that has udders and produces a little bit of milk.

Perhaps it is a Tumtum or an Androginus, which has some physical features of both sexes.

be'Virchas Kol Tuv

Eliezer Chrysler.