Not a sw but the Missouri boat ride scene in Josey Wales with the horses swimming also comes to mind. (Spit!)
But I think it looks more painfull for the guy falling of the horse!
On the “Major Dundee” (extended edition) DVD, there is a vintage featurette on some of the stunt-work called “Riding for a Fall”. It spends the whole time talking about how dangerous it is for the stuntmen while the horses don’t even get a mention.