The lives of seven ponies were placed under threat when thieves stole electric fencing from a field on Saturday.
The Exmoor ponies were grazing on land at Ditchling Common Country Park near Burgess Hill when the thieves struck.
They stole £450 worth of fencing and electrical equipment and left other bits of broken fencing strewn along the ground.
Monty Larkin, of the Sussex Pony Grazing & Conservation Trust which owns the ponies, said: “This action could have resulted in life-threatening injuries to the livestock present from the lengths of loose wire.
“If one of the links got caught round their legs it would have been like a cheesecutter and the ponies would have had to be put down.”
The Pony Grazing Trust, which provides a specialised conservation grazing service to land managers of wildlife sites, is contracted to help manage the country park by its owners, East Sussex County Council.
The seven ponies on the site are among 85 semi wild ponies which the trust has rescued from Exmoor - where some end up slaughtered.
Said Monty Larkin: “Some of them would have ended up as meat at zoos.”
The trust’s other ponies are at grazing sites throughout the county including Ashdown Forest, Chailey Common and Birling Gap.
Ironically, the theft of the fencing coincides with the launch of the pony trust’s Christmas Appeal to raise £4,ooo to fund the purchase of extra electric fencing to enable the more efficient use of its ponies on other Sussexsites.
Meanwhile, the trust is now appealing for help to trace whoever stole the fencing and left the ponies at risk.
Monty Larkin added: “If anyone has any knowledge about this crime, please contact Sussex Police.”
Work of the conservation trust itself ensures the viability of the endangered Exmoor breed whose characteristics, say experts, make them ideal for conservation grazing in both their grazing habits and their ability to survive on poor land in harsh weather.
Conservation grazing is a method of management now in widespread use across the UK and increasingly around the world.
It replicates former pastoral farming practices which until the early-20th century maintained many of today’s species-rich wildlife habitats.
The Trust employs two part-time grazing co-ordinators who manage the grazing contracts on various sites across East Sussex, supported by some 110 volunteer ‘lookers’, who ensure the daily welfare of the ponies at the grazing sites.
The ponies also provide both a spectacle and give pleasure to many visitors to the countryside.
Organisations that regularly use the trust’s service include Natural England, local authorities, the Conservators of Ashdown Forest, Defence Infrastructure Organisation (MoD), National Trust, the RSPB, Sussex Wildlife Trust and private landowners.
Anyone wishing to donate to the pony conservation trust can find more details by visiting their website at www.sussexponygrazing.co.uk