Hey Bethesda, since your adding mounted combat in 1.6, why dont you fix the current bugs you have with Shadowmere before you release a new addition to mounts. Shadowmere stopped following me for just entering a dungeon and coming back out, and there wasnt even an enemy around when i left him. I was so pissed that i lost him that I went back 9 hours ingame time just to get him back. I enter Ysgramors tomb to do the companions quests ALL OVER AGAIN, and i come back out and HES NOT THERE AGAINNNN!!!
Fix your [censored], like you have a ton of bugs in this game yet your adding more and more stuff with Dawnguard.
Your starting to act like Blizzard Entertainment, releasing games with incompletion and bugs galore.
Your better than this, so quit worrying about the overrated Elderscrolls online and clean up the mess in your most stunning game to date

I've lost Shadowmere too, but i think its because i bought another horse (which died almost a few minutes later).
People kept losing their followers in New Vegas, so the devs put in a mini ingame terminal that solved the problem. Maybe something like this could be done?

9 hours?
You should save more often...

I've never lost him to a bug....I've lost him to leaping 100ft off a cliff to join me in battle, and then in this save while I was heading to the blades temple, he decided to go an attack the nearby Forsworn. Took out 8 of them including a Briarheart. But alas he was filled with arrows and died bravely. This horse I have from Riften now isn't up to his standard...very sad.