LONDON — Today we consider the unfamiliar horse.

This afternoon, at the halfway point of the unusual and grueling Olympic ordeal that is the modern pentathlon, after the competitors have dueled with epees and swum 200 meters, they will arrive in leafy Greenwich Park to be confronted with the most irksome and potentially disastrous element of their day: riding a horse they have only just then met.

All they have to go on is riding experience, cursory scouting reports from their coaches and an official handout with equine-temperament descriptions so vague is it unclear whether the athletes are supposed to ride the horses or date them.

“Careful, very willing,” such descriptions read.

“Forward going, sensible, experienced.”

“Sensitive.”

“Easy.”

The riders then have a critical 20 minutes to get to know their horses, to learn if they are bossy or open-minded, whether they like a kick now and again or want to be left to their own devices. There will be a little sweet-talking, a few jumps and, according to Donna Vakalis, a Canadian pentathlete, perhaps a few surreptitious breaths into the horse’s nostrils as a gesture of understanding. Then athlete and horse will head off together on the hope that the date does not end in disaster.

“Your goal is to ride effectively,” Vakalis said. “And just like in speed dating — I’ve never gone on a speed date but what I imagine you need to do is the same thing you need to do in your 20-minute warm-up, to really quickly figure out what the horse’s personality is.” And, as in dating, “you are not there to fix them.”

The 100-year-old sport of modern pentathlon is itself something of an unfamiliar horse to most Americans, though it is quite popular in Eastern Europe. Legend has it that Baron Pierre de Coubertin, the father of the modern Olympics and an ardent romantic, selected the five sports based on what any self-respecting soldier would do if he were behind enemy lines, that is, repel his antagonists in a fencing match, swim across something, run a certain distance, shoot at some people and ride away on whatever horse he happened to come across.

“That’s nonsense, really,” said Andy Archibald, a historian of the pentathlon and a member of Britain’s gold-winning team in 1976. “His selection of sports was quite random.”

In the first pentathlon in 1912, the athletes were allowed to use horses with which they were acquainted. That is the case in other equestrian events, where the long-forged symbiosis between rider and horse can be the deciding factor in a competition, and the course is designed to test the horse as well as the rider.

But de Coubertin believed from the beginning that the real test of an athlete’s mettle was his ability to handle a horse that he had never met, or, in the usual lingo, the unfamiliar horse. And from 1920 onward, that is how the event has been conducted.

The process of assigning the horses is fairly straightforward. The athletes will line up at the site of the riding event and the numbered horses will be paraded before them. The athlete who is highest in the standings will pick a number, and from this one draw the horses will be assigned to all the competitors. Sometimes they might draw a piece of paper out of a hat, sometimes the ceremony involves one of those bingo-hall contraptions with the frenetic Ping-Pong balls. At these London Games, the athlete will draw from a bowl with individually numbered wooden blocks.

Someone from each team, a coach or sometimes the athlete herself, is likely to have recorded video of the horses at a test run the day before, and after the drawing the competitors will rush to study it. But it really comes down to the 20-minute icebreaker. Still, there is not much even the best rider can do about the horse that is simply bad, which is more often the case than one might expect.

“I got thrown off in Moscow in a competition last year and broke my arm in two places, and I’ve had horses that wouldn’t go over anything,” said Margaux Isaksen, a U.S. pentathlete. “I’ve been to some countries, and I won’t give specifics, but the horses are just about lame.”

The horses are often donated by show-jumping trainers, and choosing the most suitable ones can be something of an art, explained Philip Harland, the riding director for the London Olympics. Over the past few months, Harland has winnowed a pool of 149 horses to 45, favoring the capable and even-tempered.

“We don’t want any horses with any vices,” Harland said.

Stories of bad draws are legion. Many end with some successful competitor plummeting in the standings after trying vainly to negotiate with a horse that seems unfamiliar not only to her, but to the experience of being ridden at all. Considering that these are top-level competitors, there are an unexpected amount of injuries.

“Every single time I’ve been in a pentathlon, something unfortunate has happened,” Suzanne Stettinius of the U.S. team said.