I remember when my high school friend Susan had a summer job walking hots at Keeneland, Lexington's jewel of a racetrack. It was the first I'd heard of this practice, and I immediately liked the term. It was pithy, and it required insider knowledge to understand.
"Hots" were thoroughbreds who'd just had their morning work-outs, and hot walkers were the ones who lead them around the ring or shed area until they cooled down. Hot walkers hold the animals while they are sponged down, then walk them some more. Thoroughbreds get sick if they decelerate too quickly. Unlike humans, they're not allowed to go from 60 to 0 without proper attention.
Hot walkers are usually novices or interns, those on the lowest end of the thoroughbred-care team. It's their job to slow down high-strung animals who are bred to run — and it must be both boring and terrifying.
Much easier to walk hot than to walk hots.