I think 'sheep' is the only animal noun that can't take the -s morpheme for plural, but I won't swear to that.

Game animals generally take zero plural: two deer, two partridge, two snipe, two moose. But farm and other non-hunted animals: two rabbits, two voles, two horses, two eagles. It occurs to me now you also use -s with sex-specific terms: two harts, two bucks. (I wonder, did they not hunt ducks in England?)

Fowler says somewhere that this tendency can be carried too far: saying two elephant may go down all very well in the Travellers Club in Mombasa, but the rest of us say two elephants.