I know the last time I used Shadowmere it didn't glitch away, I rode her to the Dark Brotherhood Sanctuary when it was being ransacked (felt like an extremely long time ago), but she's nowhere to be found. Did Dawnguard do it, did it glitch while I wasn't present, or is there like a spell or shout that makes it appear?

I haven't had dawnguard yet, but if it died, then it will disappear for a while, then return in the same spot it died. Hope that helps.

How do I know where/if it died? I don't think it was even a little hurt.

I hear it disapears if you ride a different horse. Thus if you ride Arvak, Shadowmere goes bye bye. But if you have Arvak, then Shadowmere is obsolete.

Shadowmere dissapears... Alot of people are saying you have to go back to where you last dis-mounted shadow but that's false.. I went back to the exact place, waited 10 days and went to every stable and no such luck.. Shadowmere was great.. Arvak is convenient b/c you can summon it and you dont have to do The quest for shadow, but shadow was a beast when fighting along side of you, Arvak svcks.. two hits and it's dead

My Shadowmere also disapeared so your not alone, can't wait till Arvak.
Also can your horse die if your riding on it?

Yes! I want my Shadowmere back >( Since I got back from the Soul Cairn with Arvak I haven't been able to find him. Arvak is neat and super handy, but Shadowmere was a beast! fighting beside you and killing just about everything. Such a good and faithful steed.

*cry*
*sniffle*

COME BACK TO ME SHADOWMERE!!

Yes, actually. There's also a good chance that you'll die in the process, too, since you are a sitting duck while your character tries to get back up, which takes forever by the way.

I thought I lost Shadowmere after riding Arvak and returning from the Soul Cairn, but I finally found him. He was in the last place that I dismounted him before going to the Soul Cairn.

IF you're on the Vampire side it's safe to leave Shadowmere parked at Castle Volkihar.She'll happily stay there even if you fast travel away and ride other mounts.If you want her to tag along, just remount her before travelling.

This happens to me all the time. Usually I find Shadowmere where I last put him. Or sometimes after the horse I bought dies, he will later respawn again at my side when I fast travel.

Here's a real glitch:

I took Shadowmere to the College of Winterhold to visit my Arch-Mage Quarters. When I went back outside, I found the rear end of Shadowmere stuck in the ground, while the other half was in Faralda's hands as she ran away screaming "Mine, all mine!" He reappeared 150 game hours later, back at the College. Faralda, however, did not reappear after I beat her in the head with my mace.

Did u check for shadow up at the new db sanctuary in dawnstar?
I had I'm return there several times on my assassin char.

My Shadowmere disappeared wayyyy long time ago..... But now I have Arvak.

my shadowmere went away after i got dawnguard and i miss him. arvak can stuff it as far as i care.

Yea, I'm kinda "meh" about Arvak. It was a neat idea and summoning him is really handy, but over all I prefer Shadowmere.

UPDATE!
SHE CAME BACK TO ME!!! *tears of joy*
Well, actually she was right where I left her when I downloaded Dawnguard. At the time I was wolfing around an occupied keep near Riften and she got hung up in a little patch of raised ground. I'm guessing that's why she didn't follow me when I fast traveled away. That, or it was when I ran into the Whiterun Hold as a wolf for the hell of it before I eventually shifted back.

Either way....SHE'S BACK!!! =D

[edit]
Off topic but just wanted to add that with the Expansion I've clocked right around 4 days (in-game time) running around as a wolf, and I had to wait a total of about 9 hours (in-game time) before I shifted back! That fully loaded perk tree ROCKS! =D
...but now I'm a Vampire Lord on that toon and still loving every minute.
/End derail