Thieves cut through a 12,000-volt electric fence to steal eight miniature horses and foals.
The five horses – the largest of which is about 0.8m (2ft 8in) in height – and three foals were taken overnight from a field near March, Cambridgeshire.
One of the foals, which is only three weeks old was taken without its mother, and will die without specialist care, owner Daniel Everett said.
The youngest foal will die without its mother, its owners said
Cambridgeshire Police said the horses were taken sometime between 10.00pm on Wednesday and 07:30am on Thursday.
Mr Everett and his wife Jo had 30 miniature horses and foals which they breed for showing.
The animals are kept in a field next to their house in the village of Stonea.
Mr Everett said he believed the thieves parked a lorry on the road, cut through brambles then made their way down an embankment to the field.
“They left rubber gloves and ropes behind, so they came prepared for the electric fence,” he said.
“It’s not weaned and without its mother it will die,” he said.
Mr Everett said he and his wife “spent years creating our own bloodline”.
“We are gutted. It’s pure greed. There’s no value in the horses without the legitimate paperwork, which we still have,” he said.
The couple has informed horse sale companies and posted about the theft on buying and selling sites such as Gumtree and Preloved.