I realize that Shadowmere is pretty invincible...except from gravity. I was running away from Sabre Cats and I randomly spaz out and press Y right before a cliff...Shadowmere cushioned my fall...but, he died. So I was wondering..does he ever respawn or anything?

I think I read somewhere that Shadowmere respawns where he (/she?) died in like a week or so. If not it may have been a month and where you originally got him.