I, too, find the tail to be exquisitely done. I dont' think I've seen anyone here do a better tail.
The only thing I can see is that the bridle might use some minor shading; it would not be a flat white like that on all parts becasue it wraps around the horse (show roundness with shading) and is affected by light and shadows , as is all the horse.
How big is this drawing?
ETA now that I see the ref. Photo, I can see that the bridle has NO shading. Nevermind!