Carrot, one of the two horses rescued in a campaign started by H&H forum users, has been put down.
The horse was believed by vets to have been more than 30 years old and had a heart problem.
Carrot and Spud were found starving in a field in Lincolnshire in September 2009.
In June this year Spud was put down following several bouts of colic caused by internal tumours.
The horses were rescued by welfare charities from a field in Stamford, near Burghley, after a concerned neighbour posted photos on the H&H forum.
In a matter of hours the forum had nearly 40 pages of comments from H&H forum users desperate for something to be done to help the horses.
In January 2010 the owner of the horses signed them over to the RSPCA, and a permanent home was found for them at the Bransby Home of Rest for Horses.
“Bransby is very sad to report that we have had to say goodbye to our much loved resident and adoption star Carrot,” said a spokesman.
“Carrot and Spud both made a fantastic recovery and were able to enjoy life again with all the special veteran care they deserved.
“Despite his advanced years, Carrot was a very characterful pony; bright and sociable and not always as co-operative as our staff might have liked!
“He had been full of life and character since his rescue. In recent weeks, however, staff were concerned he was starting to lose weight and showing signs of struggling to get up after lying down.
“Our vet examined Carrot and discovered that the left side of his heart had stopped working. There is no treatment for such a serious condition and to ensure that Carrot was not put at risk because of his failing heart, the decision was made to put him to sleep.”