The background is filled with trees of Autumn, a nice river flowing down the middle of it. Maybe even a waterfall in the background as well. Leaves can be falling all around, drifting into the river or landing on the forest floor. If a waterfall is possible, even leaves drifting down the waterfall would be cool too. Maybe even there could be a nice glow coming from the back of the waterfall, like a shine for example, indicating that another world could be hidden behind it, where the King/Queen of Autumn comes from.

The King/Queen of Autumn will have a nice flowy mane and tail, leaves stuck in it. The horse has a nice pair of deer antlers, maybe some moss hanging from them, or not, whatever looks better. He/She could be wearing a crown with leaves, small pumpkins, acorns, and even mushrooms. The coat of the horse can be a nice variation of bay, palomino, or buckskin, the pattern a simple Tobiano or Frame. There could be simple jewelry of silver or gold adorning the body and perhaps mane and tail. Maybe a transparent cape thingy if it's for the the Queen.

The stance of the King/Queen is of Autumn could be walking across the river. Or it could even be stopped in the middle of it, ears perked forward, or one forward and one tilted to the side, its face pointed towards us as if looking at something.