Gem! So happy you're hosting this again, it is amazing.
I have three ideas. I have done my best to get the inspiration flowing with some descriptions, but of course - if you select one of these ideas, do what you must to make it work! Good luck everyone, and thank you Gem!

For this idea, imagine looking at an image of Planet Earth. The horse (any type) is made up of greens, blues, browns, and greys (any base color, maybe white, or a cremello). On the hind quarter and topline, you can see shapes that resemble continents. For the background, I would think something rather dark, galaxy, or constellation related.

Imagine a post-apocalyptic world, where humanity has vanished and nature has started to regain its grasp over man-made architecture. Think of vines, overgrown greenery. Somehow, equines have transformed and evolved to adapt to this new world. A robust grullo/dun/dark grey, draft-like horse emerges from this overgrown landscape, its mane, tail, and topline are covered in greenery or vines. The horse's body, striped with either silver or gold accents, resembling a brindle pattern. I think this horse would also have some sort of horn or antlers.

Idea 3: Desert Arabian
A patterned (any base coat color) Arabian stands in the desert. The sun has set, leaving the sky glowing in pink, orange, and purple colors. The horse is adorned with a shimmering golden headpiece (or crown?) and jewelry across its body. A long mane and tail with braids, with some feathers from birds woven in.

I Honeslty love realistic and Fantasy drawings so maybe a realistic drawing of a draft or sports spony moon unicorn or pegasus in the forest at night with an owl soaring through the sky. While the horse is like shades of blacks and dark blues. And the moon is a Cresent shape with tones of clouds and glowing fireflies. And the horse has an elegant long mane and tail and they could also have a Cresent mark on them some were like their shoulder or flank.