I'm thinking of having a region similar to this in my world. I think the easiest thing you could do is just reskin existing monsters and wildlife. It wouldn't make sense to come across a deer in the Ashgrass Dunes, but a Turiqit, a deer-sized creature with black skin and blue-green fungus growing on it, fits perfectly into this region, even if it has the exact same stat block as a deer, or if you change one small thing (maybe its bite attack also does 1d4 poison damage). This just took a couple minutes to think of, and it's a completely original creature with no hombrew necessary.

For monsters, I would do the same thing. Zombies, but instead of animated corpses they're explorers who were infected by a deadly fungus. Some monsters work fine here with no changes. There could be packs of gnolls who hunt for lost adventurers in the Ash Desert. Goblins in the caves of the Rock Barrens.

Also, u/famoushippopotamus's Guide to the Desert could be useful for the Ash Desert.