Ride over to Marshall to start the mission. After arriving at the marker, talk to someone named Anderson Castro to continue the mission.
There are three marker points to search out where the Sheriff is located. Once you have found the Sheriff, you will start another task to hunt down a Feral.
A Feral is actually quite easy as long as you are stocked with enough items. Once you've found its location among the marked areas, stay on its toes and in your face at all times. Keep hitting it with a good blunt weapon and keep replenishing your health and stamina.