We had that happen to one of our horses, Reno, a really sweet ranch gelding.
The spring fed pond was one they drank from all year, had barely 2" deep water and there was a large trough with piped in water right by it.
Some horses pastured there would drink from the pond and then walk to finish drinking from the trough, or walk directly to the trough to drink.
In the winter we had a heater on the trough, so that water was always open.
This one morning as I went to check one horse was in the frozen pond, swimming around and water up to his jaws!
Seems that he may have broken thru the ice, trashed until he had dug a deep hole and was stuck in there.
I ran to the barn, called vet and the local cattle feedlot, that said they would send help and grabbed a halter and ropes.
I ran back and, laying on my belly on the cracking ice somehow managed to get the halter on him and kept breaking ice as I backed up so he could get closer to the edge.
Then two feedlot people came and the three of us helped pull him out by the halter and with a rope around his behind, just as the vet got there.
Horse was in shock, staggering from exhaustion and shivering from cold.
We were afraid he would tie up, helped him into the barn, dried him with towels and blankets while vet was getting IVs in him, put a blanket on him and he finally started to come around.
After a while his legs started swelling and looked like stovepipes.
It took a few days of intensive care until he was finally well.
It too was shaking when it was over, very scary times.
We used that pasture for 100+ years without any trouble, thru many winters.
What happened was strange, this was Reno, with the English saddle, later that spring, fully recovered:
