Pit ponies were animals that were used to assist miners underground. Not all were ponies - draft horses were used in larger mines in continental Europe and the Appalachian miners, perhaps unsurprisingly, preferred mules.
True pit ponies are best documented in the UK, where they were in use from 1750 until 1999 (yes, 1999 - the very last pit pony was named "Robbie") with the highest number being in 1913, when there were 70,000 - mostly Shetland and small Welsh.
Some mines even bred the ponies underground - some of these ponies literally went their entire lives without seeing the sun. The ponies were well cared for, and generally retired to the surface in their late teens - although as they had spent their entire lives stalled underground, the poor things did not know how to be horses at all, and a lot of them were slaughtered. Also, they tended to get black lung and other respiratory problems.
Britain's last surviving pit pony, Pip, died in 2009 at a museum where he worked as a four hooved docent, demonstrating the harness he used to wear. (He has been replaced by a younger animal purchased for the job).
As far as I can tell there are no pit ponies left. But one could imagine in a fantasy world dwarves using them - and perhaps those ponies would be as adapted to underground life as their handlers.