The long-secret diaries of Genghis Khan, revealed to your racing correspondent this week by an aging jockey who said he found them in a cave near the Mongolia-China border, contain evidence that suggests the marauder has inside information about this Saturday's Kentucky Derby.

"Wild horse. (Genghis wasn't much on sentence construction.) Cavalry no good. Dogmeat. Glueboxes. Me need wild horse for wild deeds. My kingdom for a wild horse. Marfa, wherefore art thou, Marfa?"

Okay, so you think the diaries are something your racing correspondent made up in order to begin a column on Marfa, the wild horse. Actually, they are made up to begin a column on several things. We'll talk about Marfa (who, on the advice of attorney Edward Bennett Williams, isn't talking). We'll also visit Luv a Libra's lavender-fingernailed owner. And we'll talk to Eddie Sweat, the famous groom, who says horses know when they win and when they lose.

Marfa ought to win this Derby. Maybe 10 of the 20 entries can win, depending on how the rush-hour traffic sorts out. But Marfa looks the best. He is bred for the mile and a quarter. He has the big-run-at-the-end style that produces Derby victories. And he has demonstrated, perversely but convincingly, that he is the strongest colt in a crop of 98-pound weaklings.

The problem is, the horse runs crooked.

Once Marfa gets near the front of the pack, he veers to the left. The horse people call it "lugging in." Not even the horse sages know why a runner does that, except to guess that once the horse passes everybody he loses interest and wants to go get an ice cream cone.

The first hint of Marfa's left leanings came in the Spiral Stakes in March. After circling the front-runners, Marfa wanted a fudge ripple, two dips, so badly that his jockey, Jorge Velasquez, had to jerk the colt's head sharply to the right. This he did twice, the horse slowing down each time.

Finally, Marfa straightened out and, revving up again as few horses ever could do, won easily. Last month's Santa Anita Derby, another easy victory, was accomplished with only a hint of trouble. Marfa drifted left but without slowing down and without bumping anyone.

No such luck two weeks later in the Blue Grass, his last race before the Derby. Marfa that day was Genghis Khan's kind of horse. Not once but twice, Marfa crashed left into other horses. Velasquez, yanking on the reins, again had Marfa's head turned sideways. The horse's ear was facing the finish line, not his nose.

Still, a strong second-place finish led to the conclusion that if this horse can run so fast sideways, he'll outrun Amtrak if he ever goes straight.

This horse also bit his stablemate Total Departure.

As they walked off a van, Total Departure made the painful mistake of walking in front of Marfa.

"Marfa take no prisoners. (Back to the diary.) Marfa and me rule the world forever. Marfa the Conrad Dobler of horsedom."

Marfa's trainer, Wayne Lukas, grew testy early this week when reporters grilled him about his colt's misbehavior.

"These horses aren't born knowing how to run races," Lukas said. "That's what trainers are for. When this horse was a yearling, you couldn't turn him in a 40-acre field. You couldn't put a saddle on him. All horses, the same thing. You have to teach them. Marfa's still learning."

Then Lukas suggested that if sportswriters were so smart, they'd be training horses instead of asking questions that trainers don't like.

"I don't mean to second-guess you, Wayne," a sportswriter said, second-guessing, "but have you considered using a runout bit on Marfa?"

The mechanics of a runout bit put more pressure on a horse's mouth. Next to a regular bit, a runout bit is power steering instead of manual. For horses that don't go straight, a runout bit is optional equipment often used.

"I don't gimmick my horses," Lukas said sharply.

Wherever Marfa finishes, it will be in front of Luv a Libra, the horse owned by the platinum blond with lavender fingernails, Vivianne DeCosta, who drives a Cadillac with a Gucci interior, wears a $4,000 watch and says her next good horse will be named Lover or Libra.

"I'm a Libra," DeCosta said.

Unfortunately, her horse is an Aries, whose rising moon is out of synch with the seventh house of her platinum sun, and Luv a Libra can win the Derby only if Marfa bites its hind quarters all the way.

Human beings always are wondering if horses are very smart, although no one has ever seen a horse lose his shirt betting on Tree Rollins, and the question came up again this morning when someone asked Eddie Sweat if a racehorse knows the difference between winning and losing.

"Sure they do," he said. Now, Eddie ought to know. Thirty years on the track, he has been a groom with five Derby horses: Amberoid in '66, Dike in '69, Riva Ridge in '72, Secretariat in '73 and Current Hope this time.

"Horses get all upset when there are other horses up front of them and they can't catch up to them," Sweat said. "We ran Lord Darnley and Escapade once as an entry. Escapade finished so far behind Lord Darnley, like 12 lengths, that he couldn't see him and he thought he won.

"He got all wound up because they didn't take him to the winner's circle. You could see it. I was walking him back to the barn and he didn't want to go. All the rest of that day, you couldn't get along with him."