A piebald gypsy cob was found dumped on a country lane in the village of Langwith, in Nottinghamshire on 19 June.
The mare was found by local walker, Sarah Betts, who reported the incident to the police.
The RSPCA sent an officer, but does not know where the mare came from or how she died.
In the absence of an owner, the responsibility of removing the horse lies with the landowner, or the local council depending on whose land the horse was found.
Sara Howlett, the RSPCA’s press officer said such cases are becoming more and more common because it costs a lot of money to put a horse down.
“The RSPA come across far too many dumped, neglected and suffering horses,” she said.