You're correct, Tinylily.
OP- your sketch is very cute, and theres some potential. I would work at giving your shading a 'softer' look to it, and sort of blend the musculature in with the rest of the body. You want the muscles to be well defined, but not overbearing. Beautiful
work with the tail, it's quite a bit better than the sorry excuses for mops that I string onto my sketches!
IMO I think it would be a good idea to take a few horses and really study their porportions. In the few years that I've been sketching, I've developed something that I call the 'rule of thumb.' Not exactly it's original meaning, but something that has helped me tremendously in my drawing and painting. All you do is take a simple picture of a horse (I like to use illustrations with good porportioning, so I'm not distracted by backround or anything) such as this
and I use my thumb to correctly porportion everything. In humans, there is an exact symetry to our anatomy, that makes us all look the way we do, and not wonkey with huge heads and tiny bodies. i.e; our head is the size of our forearm, three times around our thumb is our wrist, our arms are roughly the same length as from our foot to our knee....etc. Theres a symetry similar to this with horses. I've learned that generally speaking- a horse's head is roughly the same size as from it's wither to it's heart girth, he's as long as he is tall, the head and neck are about the same length, the muzzle and ear are the same size...etc. You can use your thumb to 'compare' these by grazing it over one and bringing it to the other, to make sure their sizes are correct. I'm not sure if that made sense or not though, since I'm no good at putting my thoughts into words ;)
I'd also work on fine lines on the face, because those are what really give the animal its character. The shape of the eye can completely change whether it looks kind or mischievious, it's nostril size will make it look winded or calm, etc. The veins and nerves will stand out more on a hot blooded horse, while they are more discreet on a draught or warmblood. All of these things really make an sketch come alive, and give it character.