It's been a while since I drew horses, but I will surely give it a shot!
Here we go!http://i48.tinypic.com/elelut.png
my redline was kind of hard to see, so I went over it with a burgundy-line! C:
Basically, I think your main problems were these two things:
1. Proportions. The legs and head of poor Seph are much to small for his body, and this is mainly what is making your drawing look 'overweight'. If the legs were longer, then the width would be fine, but because they aren't, it gives them the appearance of being short and stubby.
2. Weight. Even though the character is flying, he still has mass and weight. Right now, the angles of the legs and wings are a bit awkward, and this makes it difficult to 'believe' what you are seeing. If he were really up in the air, his legs would be tucked in, but not all the way, because he would probably be relaxing them slightly (after all, he's using wings, not legs, to propel himself!). Also, the place where his wings are attaching makes them look kind of pasted on top of the picture. I would move them up, and really let him extend those beauties!
There are some other, smaller things that I would like to point out as well:
-The front legs look like they are attached at different points on his body, as if one was further forward than the other
-The directionality of the mane is kind of unnatural, because if he was flying forwards, then the mane should be streaming backwards through the wind.
-This is just stylistic, but if I were you, I would switch the position of either the front or back legs, because right now he looks like he is pacing- an unusual gait for a horse (unless perhaps they are racing sulkies...)
It may help you to use a reference picture next time, and circles at the joints may be useful too.
I hope this can help