The pilot stated that the flight was in support of a Department of Natural Resources (DNR) pronghorn animal capture project. During takeoff, the pilot noted that the cockpit gauges were in the green range an indication that the helicopter had enough power available for the weight and density altitude he was operating. The pilot began herding operation, by maneuvering behind a small group of animals and successfully directing them toward the trap configured on the ground. After about 20 minutes, a large group of pronghorn were found and again the pilot maneuvered the helicopter behind them in an effort to herd them toward the trap's funnel. The animals suddenly reversed course and ran directly under the helicopter. In response, the pilot maneuvered the helicopter backward about 50 to 100 feet while applying forward cyclic and maintaining an altitude of about 15 feet above ground level. The helicopter began to settle with power and as he applied up collective, the descent rate increased. The helicopter landed hard and incurred structural damage to the tail boom and bulkhead. The pilot reported that there were no preimpact mechanical malfunctions or failures with the helicopter that would have precluded normal operation.