I'm at work at the moment so can't go hunting for any examples right now, but basically practicing as many eyes as you can will really help. Just go onto google, and type on 'angry horse' or 'scared horse', 'happy horse' etc. And you'll get some hits of horses with different expressions. The eyes are the 'window to the soul', any animal is the same, you can determine their emotion by looking at their eyes.
Study the eyes closely, instead of looking at them just as another bit to draw, another shape and clump of shading, try to get to know your subject, but observing their eyes.
It is vital to use a broad range of tonal values when drawing the eyes, there will always be a '10' value (black) and always at least a 3, but usually 1 (white). I try to put in my 10, and my 1 first, and work the rest in after. Without those tones, the eye will be flat, dull and expressionless.
The same principal applies to drawing a three-dimensional sphere. The shadow is a 10, and you gradually work up to a 1, to create the 3 dimensional effect.
An eye is the same, you need to have a source of light shining off the eyeball, which creates the 10 value shadow, and the 1 value shine. This will make the eye 'pop' and appear spherical. The pupil is usually around an 8 or 9 value, I don't tend to make them a 10 unless in shadow.
The skin and fur around the eye is also important, the 'eyebrow' angle and depth enhances the quality of the emotion in the eye. So make sure you get your proportions right there. Generally there is also a rim of 1 or 2 value 'shine' around sections of the eyelid - make sure you draw this in, it is VERY important!
I hope that has helped you a little, if you feel brave enough, go away and practice just eyes, and put them up here so you can get some help on how to improve them :)