On December 10, 1999, at 1730 mountain standard time, a Cessna 421C, N3912C, operated by Flight Line Aviation as an on demand air taxi, sustained substantial damage when it collided with a deer during landing roll at Campbell County Airport, Gillette, Wyoming. The commercial pilot and his two passengers were not injured. Dark night visual conditions prevailed for this charter flight operating under Title 14 CFR Part 135 and an IFR flight plan was filed. The flight departed Rapid City, South Dakota, at 1645. According to the pilot, shortly after touchdown during landing roll, four deer appeared on the runway crossing from right to left. One deer collided with the right forward portion of the fuselage and right propeller. Both hull and propeller damage resulted. Published airport information lists deer and antelope as possible obstructions. There is no anti deer or antelope fencing or other game discouragement devices.