The British RSPCA is seeking information about the dumping of a piebald mare and her foal, both of whom died during birth.
The horse and her foal were dumped at the side of a slip road off the A13 near Stanford-le-Hope in Essex.
The bodies were found by police on Tuesday afternoon, lying on the verge of a road leading off the B1007.
RSPCA Inspector Steven Craddock said it was thought the pair were dumped around dinner time on April 22.
“It is unbearably sad to see an animal given so little care that they die giving birth – the mare must have suffered greatly and we would like to find the person responsible,” Craddock said.
“If anyone saw a vehicle or horse box in that lane around that time and took a note of the registration number we would like to speak to them.”
Anyone with information can call the inspector information line in confidence on 0300 123 8018.