The photo, taken by a passenger on a bus forced to stop by the pack, was posted anonymously Saturday, the Daily Mirror reported. It was taken in Abergwili, near Carmarthen.
"Hunting dogs came running down from the forest on the hill on to the busy road with a fox and started tearing it to pieces," the poster said. "It was one the most horrible things to experience. If you know who the dogs belong to, contact police. These people need to be sent down."
Using dogs to hunt foxes and other animals has been banned in Britain, although hunting with hounds still has defenders.
Lee Facey, master of the Joint Carmarthenshire Hunt, said the hunt's hounds were 20 miles away from Abergwili at a meeting monitored by police and animal rights activists Saturday.