Except that male beavers aren't called bucks, and the term never seems to have been used. All beavers have teeth with some similarity to what are called buck teeth in humans, but that doesn't seem a very probable reason for calling currency with a picture of a beaver a buck.

Far more likely would have been that the term was already in use informally, perhaps related to the use of deerskins as a medium of exchange in trade.

As for the objection that that would imply that only male deer skins were involved, it's pretty common to use the term duck and dog, for example, to refer to both sexes.