The disovery was made near Almondell Country Park.
Scotland’s animal welfare charity was alerted on October 22 after a member of public found the already deceased dog in a grey coloured bag with white spots.
Inspector June Chalcroft said, “The dog, a young female Chihuahua, was wearing a black/pale pink /grey striped collar when she was found.
“A post mortem has been carried out which concluded that she was very underweight.
“She didn’t have a microchip which since 2016 has been a legal requirement for dogs.”
June added, “Someone may be aware of a person in the area who had a dog matching this description which has suddenly disappeared.
“Information is received in the strictest confidence and can be reported anonymously.”
Causing an animal unnecessary suffering is an offence under the Animal Health and Welfare (Scotland) Act 2006. Anyone found guilty of doing so can expect to be banned from keeping animals for a fixed period or life.
Anyone with information is being urged to contact the Scottish SPCA animal helpline on 03000 999 999.