Neck is too thick at the base and not enough iris in the eyes, but the eyes depend on what you're trying to do because cartoon horses obviously don't emote like real horses. Real horses have very thin necks and the throat is not very pronounced.
Were you looking at this when you drew it? If not take a look [link]
the link won't work for me.. but no, I didn't have a reference. Hmm, you make a good point.. I can see it now that the neck is too thick. Oh well, not too shabby for a first try, anyway. Thanks for your feedback!