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These deer are definitely not Mule Deer. They have much smaller bodies, have a long tail like a whitetail but with a black tip and much bigger ears than a whitetail. This spring I saw one of these bucks with another whitetail buck. They were clearly different but were hanging out together. That leads me to believe they are a cross.
Most likely a Mule Deer/Whitetail cross. I have seen a couple of crosses out in Eastern WA where the two species mix. While it is not a common occurance I have seen more than a few. And the crosses that I have seen look alot like what you described. Where they have a big tail like a whitetail deer but with some black in it. Also their bodies are smaller than a Mule Deer, more along the lines of a whitetail sized deer. But they have bigger ears like a mule deer. Not AS big as a mule deers ears but definately bigger than a whitetail's ears. Also typically their rear ends will be bigger and bulkier like you would expect to see with a mule deer. I have not seen any muledeer/whitetail cross bucks only does so I can't speak of what the antlers would look like. As far as the hide is concerned they seemed to always have the hide and fur that looks like a whitetail not a mule deer. Hope this helps.