It’s some kind weird graphical glitch which is triggered by something and it does not go away. It can seem to linger forever and you can carry it over into delves and such.
People believe it is the new phase lost mount from the treasures achievement in Karesh. This is a pretty rare mount to see on the second day of patch considering the amount of time it requires which is likely why this glitch predominantly occurs in Dornogal where someone will always mount up to show off new things.