It's called catch-riding.
Unlike other equestrians, modern pentathletes don't have the luxury of riding their own horses. Twenty minutes before the event, riders and horses are paired up via a blind draw.
Twenty hours, let alone 20 minutes, probably wasn't enough time for Rachel Coleman to get a read on Walter.
Before you learn more about Walter, it should be pointed out that as a 14-year-old, Rachel is not expected to ride in competition due to safety concerns. Only athletes 18 and older are required. Rachel chooses to ride because she's experienced, having ridden since she was 7.
But during a competition in January at the Olympic Training Center in Colorado Springs, Colo., Rachel was the lucky rider who drew Walter.
By the way, did you ever notice that horses usually have cool names like Sunshine, Ginger, Whiskey, Thunder, Cocoa, Pokey never Walter.
As you might guess, Walter was old and Walter was cranky. He dug his front legs into the dirt and turned his neck into a slide, sending Rachel to the ground in front of 10 of the 12 obstacles during competition.
"I was pretty upset," says Rachel, who will turn 15 in May. "I know that riding is my main event and this was not one I could afford to mess up on."
The mishaps drew the wrath of a Team USA official, who pointed out that Rachel was a prime example of why there's an age limit for riding.
For the record, Walter also threw 19-year-old Team USA veteran Isabella Isaksen, sister of two-time Olympian Margaux Isaksen.
Walter has since been retired from competition.