The use of a ref REALLY shows, in a good way. Totally awesome.
Only thing I'd really point out [aside from the obvious wonky ear] is that the neck is a little too long and thin.
Looking at the Muybridge horse gait you mentioned, you can see how the neck is sorta compressed / lowered around the frame you used, and if you also look at Swarm of the Century 11:20, you'll see ponies do the same thing. Pony neck's supposed to be a little longer than it is in that particular frame I think, but you get the idea.
Course, you could argue that she's sticking her neck out / sorta jumping / etc., just wanted to point that out though.