It depends on the details of what made him disappear. He's supposed to respawn in the spot he died but it can glitch out and cause him to not respawn at all. On my second character I wanted Shadowmere. So I did the dark brotherhood quest right up until you get him (the quest to go kill cicero). After I got him I proceeded to go off and just do other stuff because I had already completed the DB on my first character and knew what happened(I never went and killed cicero). Well, bad idea. Shadowmere was killed by a dragon in the courtyard of the college while I was inside (which is [censored] if you ask me. I just came out and he was dead and there was a dragon going crazy on everything. stuff shouldn't happen in a cell that's not even loaded.) But his body layed there, right where he died, until an update came out. After that his body disappeared and he never respawned. I think the reason he never respawned is because he died while I was still technically on the quest you get him during.

My point is, that depending on the specifics of what killed him, where, and when, he will respawn or he won't. No real way to tell. It's possible that when you summoned Arvak it tagged him as your new horse and it dismissed Shadowmere. I don't know. I kinda wish I was a PC user so I could look at the code for this kind of stuff. But then again I wouldn't be playing dawnguard now would I?

If you could use a horse call or something to have Shadowmere show up, instead of always being there even when you fast travel, I would take Shadowmere hands down. I prefer Arvak only because he's easier to manage with the way I play.

Shadowmere because he can sprint for long periods whilst Arvak can't.

But the second I started the Dawnguard quest in Whiterun and I walked outside to hop on Shadowmere, he ran off. I checked everywhere, EVERYWHERE! Falkreath, Dawnstar, reloaded Whiterun, I checked both Fort Dawnguard and Castle Volkihar, I checked the College, several ruins....nothing...he ran literally just ran away.