The eyes have it to be sure. I have always been fascinated by a horse's eye. They tell you so much about them and you most definitely get a glimpse into their soul.
and when it comes to painting them - the eye is key - if you can capture their eye - you can capture their essence.
They say "The wind of heaven is that which blows between a horse's ears."
But I say that you can see the glimpse of Heaven through their eyes.