Trainer Wendy Kelly has been fined $1000 after the wrong horse was brought to Caulfield to race.

Kelly had Gibbon engaged to run in Saturday's Darren Gauci Handicap but stablemate Fast Cash was instead brought from her Cranbourne stable.

Realising the mistake, Kelly, who was already on-course after having another runner in an earlier race, immediately informed Racing Victoria stewards who ordered the scratching of Gibbon 32 minutes before the race.

Kelly told stewards the two horses were of similar height and size and each has a white marking on their noses.

"They are also stabled close to each other," she said.

Even when Gibbon arrived on course Kelly didn't realise the case of mistaken identity until she received a phone call from a staff member at Cranbourne 45 minutes before the race asking what time Gibbon was leaving for Caulfield.

"When he arrived on course he had a rug on and I patted him on the head," Kelly told stewards.

"I didn't even twig then that it was the wrong horse.

"I would have detected it if I had seen him without his rug on."

Kelly was charged by stewards under an Australian racing rule of failing to ensure the correct horse was presented to race on Saturday.

"It was an honest mistake," she said.

"It's unfortunate because I thought he had a good chance today."

Steward in charge of Saturday's meeting Rob Cram said it was a serious breach of the rules.

He said stewards had taken into account Kelly's unblemished record and that she had raised with stewards that the incorrect horse was brought to Caulfield.

"It's a good thing this has been detected," Cram said.

Kelly said in more than 20 years of training it was the first time she'd had a horse scratched because of mistaken identity.

Jockey Beau Mertens received an eight-meeting careless riding suspension aboard Run Gypsy Run in the Le Pine Funerals Handicap which commences after he rides at Flemington next Saturday.

He returns for Sandown on May 26.