The body of the Skewbald pony, thought to be only two to three years old, was discovered on Saturday evening, just off Scawby Road in Broughton Vale, Brigg.
RSPCA inspector Daniel Bradshaw said the animal showed signs of having been badly neglected.
He said: “She was in extremely poor body condition with her ribs, hips and spine easily visible. She also had blood coming from her nose and mouth and her hooves were in poor condition, which shows she was severely neglected until her death. From the tow strap left around her neck it looks like her body may have been dragged off the back of a vehicle.”
Mr Bradshaw added: “This spot has been used for fly-tipping before, but to treat an animal like rubbish is disgraceful. What a terrible ending to a very short life for this poor pony.”
Anyone with information is asked to call the RSPCA on 0300 123 8018.