A thousand feet below the surface in the dark ?
MY guess is .... the damned thing was a prison and those are the scratchings of the desperate wanting OUT'A there ... on a swift steed
LOL. Well, 20,000 years ago it may not have been 1000 feet below the surface. And they may have had a light source we are completely unaware of. That is an incredibly realistic drawing of horses for any time period.