Just a question that popped into my head after someone said this to me…
Well, racehorses are often given furosemide, which is a diuretic, so I suppose some of them probably do pee more than your average working or saddle horse.
I think it is just that the expression ‘pee like a racehorse’ rolls more easily and poetically off the tongue than ‘pee like a horse’.
Race horses are often actually trained to urinate just before a race. It’s believed by some trainers that this reduces weight and improves comfort & performance of the horse.)
So the horse often urinates when being prepared for the race, and often gamblers are watching them carefully at that time, trying to get some inside info that will let them beat the laws of probability.
Thus it’s probably more common for people to have seen a race horse urinate than, say, your average plowhorse (who does it out in the field with nobody watching). So this may have led to this saying.
I was doing a final inspection of a horse racing facility and saw an ad posted for a veterinary assistant. One of the job qualifications was the ability to whistle. I couldn’t for the life of me figure out why, so I asked the track vet. He said, “Race horses are trained to urinate on command. One, to make them lighter, and two, so we can collect samples to test for doping.”
My guess is that it just comes from horseracing. If you were a racing fan (presumably earlier this century), and watched a number of horse races, you’d become accustomed to seeing the horses take a leak. Thus, you’d see the connection between the term and the reality. Also, the fact that its a racehorse implies speed and such, which might have added to the image of them being very diuretic.
[ ANECDOTE, NOT DATA ] I remember a number of years ago the drug was Lasex, and was given to prevent nosebleeds–because nosebleeds can pretty much end the race for the mutant horsies. The lore tells that the initial dose of lasex gives the horsie a little extra mojo which can in turn impove the economic performance of fortuitous investments in the perfomance of the horsie. It also makes them pee. Do they still use it or has the drug of choice changed? [/ANECDOTE, NOT DATA ]
Lasix® (Furosemide) is given to racehorses that are certified as “bleeders” by a veterinarian. In addition, horses that run hard often do some damage to their muscles, releasing myoglobin which then is excreted in the urine, causing an increased amount of urine to be produced.
My 13 year old daughter takes all these factors into account, then picks a horse with a “cute” name. She wins more than she loses, almost making up for her dad’s miserable performance.