Very rarely, a glitch will occur on the Xbox and PS3 versions of the game, where on a clean startup of the game, all horses will appear this horse's color and after a couple seconds will return to their normal colors.
The Dead Horse model appears to be based on the Painted Standardbred.
Hey there, cowboy. It looks like this little ol' article is a bit puny. I reckon you could impress me by adding to it. Don't be shy now. If you're logged in, it'll help you earn them fancy achievements, too.