From what I can see, the highlights are Ok, so the exposure is good. The camera was probably in evaluative mode, so it was picking the exposure for the overall image.

(If the exposure was in spot metering mode, and with the horses as the spot source, then the shadow side would be lighter, but with the risk of the highlights elsewhere in the image blowing out.)

The subject itself is backlit, so the foreground side of the horses are in shadow. Typically, on the shoot, some fill flash might have evened those out a bit, but you can dodge them in post to bring up the shadows.