"The dent above the eye is called the supraorbital fossa. It normally just contains fat tissue.
It usually gives a rough indication of the condition of the horse.
Horses in poor condition (with low reserves of body fat) tend to have bigger dents and conversely, fat horses have less of a dent or no dent at all.
There are exceptions to this, eg. a horse with Cushings disease will often have bulging supraorbital fossa.
Also trauma/bruising to the area can cause the dent to fill. The fossae should be symmetrical; if they are not, then you need to suspect bruising or a growth."
My boy has really deep ones, despite being a fat git and not particularly old. He's always been the same. I think it gives him a bit of a worried expression.
Dan has really big ones, but he's always been skinny. He's put on a bit of weight since being on box-rest though, and the dents are still as deep as ever. He's the most beautiful horse ever.