SAHARA DESERT, Morocco >> The most popular runner in one of the world’s most difficult races is, strictly speaking, not an official entrant. Or even human.

A dog nicknamed Cactus suddenly appeared Monday during the Marathon des Sables, a 140.7-mile stage race made harder by the heat, wind, sand dunes, rocks, stony plateaus and dry valleys of the Sahara in southern Morocco, the rough equivalent of running 23.5 miles a day for six days.

Cactus skipped Sunday’s first stage, but ran nearly 15 miles Monday, then completed the full 23 miles of Tuesday’s stage and the entire 47.4-mile stage Wednesday, showing great resilience and a skittering ability to navigate the dunes.

While the nearly 800 human runners were given 31 hours to complete Wednesday’s ultralong stage, Cactus needed only about 11 hours 15 minutes, some of it in a sandstorm, which would have put him 76th in the stage results. Then he ran a couple of extra miles to cool down.

Officially, Cactus has run more than 85 miles from his home village, and his canine wanderings have gone viral. A leading French animal magazine has inquired about Cactus’ life story and how he is being treated. A Moroccan television reporter went directly to the source and placed a microphone in Cactus’ face. Officials are now listing three categories of leaders: men, women and dog.

“I know he’s having the greatest time,” Karen Hadfield, a hotelier of British and Australian roots who owns an inn where Cactus lives, wrote Wednesday night on the Marathon des Sables’ Facebook page.

The dog is accustomed to the companionship of strangers. He is also a nomad who often travels about 25 miles a day around the area “just for fun” in this Berber region where humans and animals are familiar with long distances, Hadfield wrote.

A standard marathon-length stage of 26.2 miles is scheduled for Friday, followed by a concluding 3.8-mile stage for charity Saturday. Today, a race spokesman said that Hadfield planned to retrieve Cactus on Saturday at the finish line.

Until then, if he remains in the race, Cactus will sleep wherever he wants at the roving bivouac where runners, organizers, support crews and journalists camp nightly in tents.

He is being given water at checkpoints on the course and food by runners and officials at various places, including the campsite. The medical crew said he appeared generally healthy.

While dozens of human runners line up daily to have their battered feet tended to after long days in thick-soled shoes and gaiters to keep out the sand, Cactus appears to be doing fine in bare paws.

“No blisters, nothing,” Anthony Serena, a podiatrist, said.

In the early mornings, Cactus makes his rounds at the runners’ tents, posing for photographs and rolling over for belly rubs. Then he trots easily to the front of the starting line and takes off with the pack as AC/DC’s “Highway to Hell” blares over the loudspeakers.

“Lots of runners are nervous in the morning, but he was pretty casual,” Meghan Hicks of Moab, Utah, who won the women’s race at the Marathon des Sables in 2013, said of Tuesday’s stage. “I think he was channeling the spirit of the day.”

Cactus joined the race Monday, near a checkpoint at the Erg Chebbi, where runners entered a sea of dunes for 8 miles of exhausting undulations. Well, exhausting for nearly everyone but Cactus.

“That dog was a beast; he ran right past me, and I couldn’t keep up,” said Theo Holzapfel, a runner from London. “I kept following his footprints; I figured he knew where the hard sand was.”

On Tuesday, Stephen Homesy of Washington strayed from the line that most runners were taking through an area of small dunes and a dry valley, then began to wonder, “Am I headed to nowhere?”

Suddenly, Cactus appeared, and Homesy followed, steered back on course.

“I don’t know why I followed, but it seemed to be a sign that he knew where he was going,” Homesy said.

Technically, Cactus failed to pay his hefty entry fee of about $3,500. And he is fresher than other runners, having skipped the first stage.

“He’s fast, but he has four legs, not two,” said Emmanuel Lamarle, a race spokesman.

Still, Valentin Campagnie, an official race photographer, placed a medal around Cactus’s neck after Tuesday’s stage. And Cactus did what every marathon runner does with his medal: walked around to show it off.

“He’s unbelievable,” Campagnie said of Cactus’ running ability. “His heart is little, not like a big human heart.”

Some were concerned about Cactus before Wednesday’s brutally long stage of 47.4 miles.

At a checkpoint after 16 miles, Cactus vomited slightly, then rested in the shade of a military truck for a half-hour in the heat. He seemed to drink little or none of the water placed at his side. But he then got up and continued on his way, and seemed fresh at the finish hours later as temperatures cooled significantly in the dark.

“No one has seen anything like this,” said Mohamed Ouhassou, a Berber and driver who has ferried journalists during the Marathon des Sables since 2004.

In the Berber culture, Ouhassou said, dogs are viewed as workers to guard goats and other animals, not as pets. If organizers of the Marathon des Sables were Moroccan instead of French, Cactus would likely have been shooed off the course days ago, Ouhassou said.

Instead, his whereabouts are relayed during each stage with great anticipation.

“Maybe next year,” said Hassan Taouchikht, another driver, “all the runners will bring their dogs.”