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.
The video is here: https://www.youtube.com/watch?v=7qxTKtlvaVE