These beautiful animals are not easy to photograph. Because they are big, bulky and equipped with a greyish skin they can end up as rock like things on photographs easily. Especially in low contrast light it is hard to get a good photo.
With regards to composition, keep in mind to photograph them from an angle, just like you would do with a horse. With regards to the light, experiment with the aperture, a f-stop higher than usual might be helpful. It can also look great to photograph them backlight, although only to a certain extent.
Photographing them is challenging, yet amazing when you get the photo right.