So I have a bit of a problem with one of my mounts, but in fact-I don't have this mount!
I got a starlight pony from one of the packs in the crowns shop. (I think it was the Ninja Lore Pack.)
It was a 24 hour mount. After 24 hours, the timer ran out.
I didn't think much of it-except I realized the notification would come every single time I went in to a new area!
For example: I was in Ravenwood. The reminder appeared. I pressed it, and clicked 'OK'. It was gone, until I went in to the commons and the notification reappeared!
I'm not sure that there is a way to fix this via Options Menu, but it is VERY annoying.