Strange Horse keeps appearing
I asked Lydia to buy me a horse, which she did. This horse had Ride Jogon's Horse (E) when approached. I used the horse to get me to Dawnguard fortress and then I was asked to find some helpers. After finding them my horse disappeared when I had to fight some bandits and I could not find it again.
Later on while undertaking another quest I came across a dappled grey horse while heading north and when approached it came up with Ride Horse (E).
I did so and rode it when going to see Serana's father. But it had vanished when I came back to the boat landing.
Later in the game I was heading down the road from Winterhold and the horse was standing in the middle of the road. Once again when approached it had Ride Horse (E). Once again I mounted it and rode it to Lakeview and put it in the stable.
The next morning it was gone and I haven't seen it since.
Does anyone know who the horse is supposed to belong to and why it can be ridden but does not stay around?