I don't know if anyone else had this happen or not, but the other day outside castle volkihar serena and shadowmere started attacking one another. It was a never ending fight since shadowmere's health regenerated fast. I watched this go on for about 10 mins before I had to reload since they wouldn't stop. Pretty entertaining actually.

Is this an issue or a notification?
if it's an issue, and you have 1000 gold to spare, buy a horse at any of the stables in Skyrim, ride the horse away and kill it. you will not have a horse on you when you spawn and shadowmere will be waiting at the stable where you bought the horse.

It's not an issue, I just thought it was funny. There was no reason for them to attack each other, they juat randomly started to brawl lol

I hate shadowmere... The last 200 years made him a bad horse

It also happens to me with serana,she keeps fighting with my summoned daedra.

How dare you
He's my second favourite horse
(after Arvak)

The only thing I really like about arvak besides how he looks, is that if he dies you can resummon him.

Agreed. They also turned her into a he.

I miss the old Shadowmere...

Both are immortal monsters...and besides shadowmere probably made a snide comment about her and molag bal