What a gutzer of a thread! In Wonthaggi and Korumburra, the pit horses ( they were full sized horses, not ponies) were brought up every day. There's no record of any going blind. I highly recommend a book called "Come here, gee off" by Joe and Lyn Chambers, available at the State Coal Mining museum in Wonthaggi, Victoria (Aus).
These horses were tough, absolutely ruthless with the miner's crib tins, could open screw-top bottles, and generally could read danger well before the miners. They kept them on at Wonthaggi because of the faulted geology which made it dificult to install powered conveyers and suchlike. Oh! a gutzer, by the way, is not only the word I use for "Immense" or "Highly satisfactory", I found it to be the word used by Korumburra miners for the huge hand-brace type drill they used to use for boring the seam to set explosives. The one I saw was made by a blacksmith. The handmade bit was about ten feet long.

The horses could count. If they heard the sound of three sets of chains taking up when they moved, they kept going, if they heard four they would stop and not move. Three skips was their allocation and they knew their rights. Good union horses, those.

Good 'ealth all. Jack Halyard