Per subreddit rules about video posts, you’ll have to search for the video on YouTube. I watched the video which was 9 hours after posting.
The experiment was done in California using a light draft horse with help from Motivo. They modified a car so the horse can pull a rope connected to wheels sitting on a dyno. Comes out a horse makes ~5.7 horsepower for the specific type of horse they had.
I remember reading that the average horse produces 4hp. It was printed in a car magazine in either the 90s or 00s. C&D, R&T, or possibly SCC. Always stuck with me.