Well, it's actually really good for a sketch, I say! I am not an expert on horse anatomy, but I know a reasonable amount, and about the only major thing I can see wrong with this picture is that the tail looks rather off-center to me; it looks like it might be coming out of one of his his butt cheeks (I can't remember the proper horsey term) rather than a continuation of his spine. The again, it might be the funny angle... On closer inspection, his hocks (I think that's the proper term - the narrow bit right above the hoof) look rather too thin on the front legs, and something about the hindlegs bothers me, but unfortunately I can't put a finger on it. ^^' The angle looks weird on the hooves as compared to the rest of the legs, or something, I think. It might also look better with some shading, but since this is a sketch that's not as important; if you want to make the picture more complete, it probably would be. This is coming from someone who hardly ever properly shades her pencil work unless it's a Big Project, so take that as you will.

Also, is this meant to be drawn in the style of the horses from the film Spirit: Stallion of the Cimarron? If it is, you've got the style down great; admittedly this is mostly only visible in the face, the rest is just normal, mostly realistic horses. Also, this just plain looks like Spirit himself, besides the fact that his mane, tail and eyebrows are black. All his other markings are there; the dorsal stripe, the colored muzzle, and the stockings on his legs. If this is Spirit fanart, you might want to give credit in your description.

If it's not, well, it sure looks like it, so in that case you might want to add a disclaimer that it isn't in the description. ^_^

Keep drawing!