Why do some horses have hollows above the eyes?

Actually, all horses have a bit of a dip above the eyes. If they don't, then they are seriously obese.

However, it is more obvious in some horses than others. The hollows often become deeper as a horse ages. Other reasons for deep hollows above the eyes are emaciation (and in some cases the hollows never return to normal when a horse recovers from being starved) or genetics - as in some horses have deeper hollows than others.

The actual purpose of the hollow is to accommodate part of the top of the jaw.

On this older horse you can quite clearly see the deep indentation above his left eye. This horse was in his late twenties (and yes, still serviceably sound) when this picture was taken.