Mortlake police Sergeant Matt Laxton said that some time between 7.10am and 9am on Friday the seven-year-old old mare, which was tethered near the intersection of Hood and Webster streets, was set free by an unknown person.

"The mare wandered up Hood Street and was struck by a passing vehicle," he said.

“The driver didn't stop, the mare suffered severe leg injuries and a passerby alerted the owner."

Sergeant Laxton said a vet attended and the miniature in-foal mare had to be euthanased on the roadside due to the extent of her "horrific" injuries.

"The seven-year-old mare is a family pet. She was in foal and fed by the nightshift working owner just after 7am," he said.

"The owner is quite traumatised. The driver would have known they hit the mare. It's unconscionable behavior to leave an injured animal on the side of the road – it's despicable."

Sergeant Laxton said the two miniature horses in a paddock near the intersection were well known in Mortlake.

"Whoever let the mare go has had no forethought about what could happen. It's just a tragic set of circumstances," he said.

Anyone with information is requested to contact the Mortlake police station or Crime Stoppers on 1800 333 000.