That's no reindeer, and it's certainly no moose. It's an Irish Elk, Megaloceros giganteous, a deer that happens to be neither an elk, nor really all that Irish. What it does happen to be, though, is long extinct. Renowned taxidermist Ken Walker has reconstructed Megaloceros from the tanned hides of once-living Canadian deer.
The mount is made of elk skins stretched over a custom foam form, and fitted out with a pair of fiberglass antlers. Using Paleolithic art as a guide, Walker also gave the giant deer a prominent shoulder hump with contrasting coloration. Walker's prowess with taxidermic reconstruction isn't just limited to extinct animals. He has also won awards for Thing Thing, a panda made from the dyed fur of other bears.
Taxidermic reconstruction occupies a particularly strange area within the already weird world of taxidermy. It uses the parts of recently deceased (but still extant) animals to create a scientifically accurate fantasia of an animal too rare to kill, or so long gone that no modern human has seen one alive. In other words, it's fake nature at its most realistic.