I've got a problem with the soil outside a stable getting eroded by the
horses.

Plastic grid tiles, which you infill with soil, gravel or grow turf
through. They're mostly sold for car hardstanding, to look better than
bare gravel, carry weight better than grass and drain through better
than tarmac. £20 /m^2 or so, Screwfix have them, or you can shop
around at real merchants for tougher ones.

I had this problem once, I had no money to spend on anything to help, so
when I mucked out I spread the dirty and wet straw on the muddy areas and
eventually it dried and made a good surface.

kate