How about a unicorn laid some eggs, sat on them for 28 days as normal. After 28 days they hatched out but one of them did not have a horn. Hence the first horse was invented.
Very plausible MissC. I was thinking along the lines of ... Herr Diesel invented the diesel engine, Herr Wankel invented the Wankel engine, so it must have been Herr Horse who invented horse power.
His name, when inverted (Horse Herr) also gave rise to the practice of mattress stuffing, a little known fact I'll grant you.