It apparently "respawns" 24 hours later at the place of death, according to UESPWiki. Which I would think is a pretty trustworthy source.
Really? That's pretty awesome then. I always reload if I accidently hit (and kill) my Shadowmere with my hyper-enchanted two-handed daedric greatsword of doom. If it does respawn, I shouldn't have to worry about that. Gotta try this out.