Shadowmere has been lost in my game for a VERY long time now. Is it possible that he is just completely gone? Or could he still be roaming somewhere? If so, are there any places I should look that he normally goes to? He's not in the spot where he first spawns outside the DB sanctuary and he's not where I THOUGHT I left him... any ideas? Thanks!

I noticed that if you neglect to ride him, after awhile he goes away. At least that happened for me.

If he died, he will be in that spot.

Sometimes she may not respawn after dying.

Look http://www.uesp.net/wiki/Skyrim:Shadowmere for other issues with her.

Is that just shadowmare or for every other quest given horse in the game?