One horse was found dead and another in an emaciated condition by an RSPCA officer responding to a tip-off from a member of the public.
A bay colt had died of starvation, while the surviving animal — another bay colt — was even thinner, when the officer, accompanied by a police officer and a representative of World Horse Welfare arrived at a yard opposite Wexham Park Hospital in Slough, Berkshire, on 2 April.
The survivor was taken away and is currently recovering in a specialist yard where his condition is described as “still pretty critical” but it is hoped he will pull through.
An RSPCA spokesman said it was difficult to give exact ages for the animals as they were both so emaciated but said that they were both yearlings.
The organisation is currently trying to find out who owns the yard and who the horses belong.
RSPCA inspector Derek Wilkins, who is investigating, said: “The surviving colt was even more emaciated than the one which died and the vet, who is very experienced is such matters, said it was the most emaciated he had ever seen.
“I would urge anyone with information about these animals to contact the RSPCA.”
Anyone with information can contact the 24-hour cruelty line on 0300 1234 999.