Red-1
I used to be decisive, now I'm not so sure...
Yeah if it’s random I would say it odd and she is probably just regretting selling the horse which could be a problem because she could ask to buy the horse back. Why would you bother posting photos now when you didn’t at the time.
I recently posted a photo of a previous horse, as my cover photo. I have also posted the same horse on here, a couple of times. It is nostalgia. It doesn't mean I want the horse back. I couldn't anyway, as it happens, the horse is not on this earth any more.
One horse I had was from a pro. He featured on the pro's web page. This is fine, he was a fine looking horse LOL.
Even if someone asked to buy back a horse I had, it wouldn't bother me unduly. I would just say no. Once, a buyer of one of mine didn't seem happy with him, I offered to buy him back like a flash. It wasn't a creepy thing, she said no, I moved on. Kept in touch for longer than the horse was alive (lived to a ripe old age too, and fulfilled more than her wildest dreams - they just had a slow start until they formed a partnership).
OP, I do think it is a more about your confidence thing. He is your horse now, go ride him like you stole him and make memories of your own. If that isn't an option, I would set your FB settings so you don't see posts from the previous owner.
I'm sure other people locally either don't know or care whose the horse is, or do know, in which case they will know it is an old photo.