Did you have a specific time period or country in mind for your question? And are you interested in just horse-riding accidents or also accidents involving horse-drawn vehicles? I own an old history of the English courts in the 1860s that lists statistics for accidents involving both riders and passengers in horse-drawn vehicles as well as details of specific incidents. I'd be glad to post some footnoted info from that source if it fits your question.

Edit: My apologies for not contributing what I could find in my books, I am moving house at the moment and most of them are still packed. I haven't been able to find the relevant one.