The world would like to know!!!

I have. Been able to ride Shadowmere around as well.

Can you please enlighten me as to how you found Shadowmere after he disappears?? I summoned Arvak, and of course, Shadowmere was gone.

I think he respawns after 30 days or so if something happens to him.

I just fast travelled several times and he showed up at the stables outside Markarth one time. I don't recall leaving him there.