Board Thread:Spoilers!/@comment-24674051-20160625215037/@comment-24674051-20160710214107

In fact, back in season 2, David presented "Prince" Henry (well, Henry is technically a prince anyway) with a young steed to start bonding with ("The Doctor"). So Henry has reason to visit the stables. which he, quite frankly, has not done in a while (and who knows what happened to that particular horse when Regina undid the curse in S3). Though I don't think Henry's was black (which the horse in this shot obviously is). The horse could also be Violet's, since part of Henry's "heroism" in her eyes was to rescue her horst (even though it was a set up by Emma).

However, the horse is more likely be for a flashback of some sort. Possibly Regina, since I think most of her horses were black.

Of course we have very little to go on (and will likely not get too much more until the end of Next week when the SDCC Panel occurs).