This sign doesn't need any explanation.
We saw at least four moose this close to the road on the main highway -- not in the park. This was one of them.
And, here is today's English lesson (YAY!) The signs are correct.
(Thank you to the Alaska State Department of Transportation for using a dictionary.)
Animals with horns or antlers usually are not made plural by adding an "s".
1 deer, 28 deer
1 sheep, 78 sheep
1 caribou, 83 caribou ("caribous" is acceptable in some