I made friends with the foresworn at Druadach Redoubt. They aren't hostile to me, but they just murdered my nice dappled horse!

That happened to me the last time I did the forsworn quest and took a horse to their cave.

Never trust their kind

Who attacked first? Them or your horse?

You see Madanach and his followers in the cave are coded to be friendly to you, its likely however your horse was not. It was pretty much exactly what would have happened had you rode your horse into another forsworn camp that was not friendly towards you. Your horse would have provoked them as well.

Your horse was likely guilty of causing the whole thing.

Do not disrespect the horse!

But, yes, you're maybe right. The horse did appear to go hostile as soon as I dismounted.

Rest in peace dear horse.

But this is why I don't use horses... Well one of the reasons anyway

Avenge him!