My first reaction to this is that it's pretty cool. You gotta wonder where a deer like that came from being that there aren't any facilities nearby that would be an obvious source for an exotic species.
I do agree with Woodsie that it's poor judgement to shoot ANYTHING if you aren't positive about what you're shooting. However, I don't think this guy would shoot a horse or a lab. Horses and dogs are species that would be easily recognizable to most people here in the US. Even being able to tell the difference between elk, mule deer, whitetail, etc would be something that most hunters could easily do. However, we're talking about a species of deer that is for the most part NEVER seen here in the wild. There would be absolutely no reason for him to suspect that a giant bodied deer out there is anything but what he's hunting. Especially when there were whitetail bucks hanging with it.
I heard a story from first session gun season in my county about a guy who picked off a coyote with his muzzleloader at about 120 yds. Walked up and realized that it didn't really look like a coyote... They called the DNR, who came and identified it as a Red Wolf... He took the animal without issuing any tickets. My point is, there aren't really supposed to be any wolves around here (even though there are... I saw one last season up close from my treestand), so the guy wouldn't have any reason to expect that it could be something other than a coyote....
I mean, the only visual difference in his deer was the size (being that whitetail deer have so much variation in actual color), so he knew he was shooting a giant deer, just not a different species.
