do you have a button on the equipper showing the background and the remove button?  I had a 'sales' background that I also couldn't figure out how to get rid of and feel like I overlooked what the horse had on in equipper every time.

I can't remember if it was in the equipper or not, since it was a personal background - like that one is.  Can you look on your horse's edit page and see if there's anywhere to remove a url?  That may have been the way I got the image off of my other horse.

That is not a game background it is a custom background. You have to go to edit their profile and at the bottom there will be a spot for custom background URLs. Remove it and save and it should be gone. May need to refresh as well. 

Hooray! There is a 'remove custom background' check box. Thank you both!