The colt - not believed to be any older than four months - was found in an “appaling condition” by a member of the public in Slayley Hill, Clowne, on Tuesday, January 26.
The RSPCA and a vet went to the scene, where the vet said that the foal was so poorly that he had to be put to sleep to prevent further suffering.
Images sent in by the RSPCA show the young foal.
RSPCA inspector Rachel Leafe said: “It was heart-breaking to see the foal in the state that he was.
“We believe he had been dumped overnight and he could not stand. When I got there, he was trying to wiggle his legs but he could not do much more.
“When he was found, a member of the public kindly put their coat over him to keep him warm but sadly he was too ill to survive.
“While there were no obvious injuries, he was very weak and in an appalling condition.
“Dumping an animal like this is just callous - the foal did not stand a chance. It was such a shocking sight to see, and the vet was shocked at his condition as well. I find it very hard to comprehend why somebody would do something like this. The foal had just been left to suffer alone down an isolated country lane.”
Anyone with any information should contact the RSPCA’s appeals line on 0300 123 8018.