Showjumper Corrine Collins had five yearlings stolen from a field of broodmares near Betley in Staffordshire earlier this month.
The yearlings, which were turned out in a large unsupervised field, were taken at some point between 7 and 19 August.
The gate, which was only padlocked at one end, was taken off its hinges.
Ms Collins said: “This looks like a professional job done by two or three people as the yearlings would have been hard to catch and load.
“I think they were caught and placed in a stable before being loaded onto a lorry.”
She said the youngsters were all young competition horses.
“We’ve kept horses in that field for a couple of years with no trouble whatsoever. This is really frustrating as they are an important part of our new breeding business,” she added.
“I can’t see them going for meat, if they did want them for that then surely they would have taken the broodmares?”
The two chestnut geldings, two chestnut fillies and one grey roan filly with bay flecks, are all microchipped.
If anyone has any information please contact PC 5697 Pope on 03001 234455 quoting crime reference number 12/56608.