I actually use a lot of defaults (Just with altered brush settings depending on what i'm drawing) for basic shading and for manes and tails! For backgrounds I use a variety of foliage brushes (Check the brushes section of my favourites, they aren't always ones I use but they are similar)
For the main horse shading I use an altered square brush, with soft edges and a speckled texture -once again with a variety of different settings depending on the drawing. I have modified it but i have no idea where the original is from (I have looked!) There are similar ones in the brushes section of my favourites though!
The brush you used in your recent painting "WS | Salute" Looks kind of similar
Hope this helps