A three-year-old filly named Ruby Queen had never won a race until she appeared to blow past the field as a $US110 longshot at an Ohio horse track.

But it turns out that she was really a he.

Track stewards suspended three people and fined another after determining there was no intentional wrongdoing in a chain of mistakes that allowed the wrong horse to run under a different name last month at Hollywood Gaming's Mahoning Valley Race Course near Youngstown, Ohio.

An investigation found that a stable worker went into the wrong stall on November 4 and brought out a male horse named Leathers Slappin instead of Ruby Queen, who was in a neighbouring stall, said William Crawford, executive director of the Ohio State Racing Commission.

A track employee, known as an identifier, then failed to properly check the horse before what was supposed to be an all-female race, he said.

The identifier's job is to verify each horse by looking at the numbers on its lip tattoo.

"It's unfortunate that it happened," Crawford said earlier this week.

Such a mix-up is rare, but not unheard of, he said.

A review of the wagering revealed nothing unusual, leading the commission to determine that the horses weren't switched to affect the race's outcome, he said.

The horse that won by nearly eight lengths was disqualified, but the error wasn't discovered until after the bets were paid out.

A $US2 wager on Ruby Queen to win paid off $US220. Anybody who did win kept their money, while those who had placed bets on the next three finishers were able to cash in if they still had their ticket, said Bob Tenenbaum, a spokesman for track owner Penn National Gaming Inc.

The company, which operates casinos and race tracks in 16 states, did its own review of what happened. The employee who was the identifier is no longer employed by Penn National, he said.

"This was a very unusual circumstance," Tenenbaum said.

"It was simply a series of errors."

The stewards suspended the identifier for 60 days and fined him $US500. The stable worker was suspended for 30 days and fined $US500. The horse's owner was for suspended 30 days and fined $US500. A substitute trainer was fined $US200.

A new layer of post-race checks will be added at all of Ohio's thoroughbred tracks to prevent a repeat, the racing commission director said.