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.
When I saw the video in my feed I immediately questioned how Mythbusters never did anything with that.
In the video he meets with Adam Savage and he says the same thing. Adam said it’s right up Mythbusters alley