The pilot reported that, during takeoff, he heard a "bang," and "it felt as though the airplane hit a brick wall." He added that, during the climb, he examined the engine instruments, which appeared normal. About 800 ft above ground level, he looked out the left pilot window and observed that the fixed left main landing gear was "oddly positioned up next to [the] seat/door." The pilot reported that he contacted air traffic control, declared an emergency, and performed a "gear up" landing at the destination airport. The airplane sustained substantial damage to the left stabilizer. The pilot reported that there were no preaccident mechanical failures or malfunctions with the airplane that would have precluded normal operation. After the accident, the following was added to the Airport Facility Directory: "Deer and other wildlife on and in vicinity of airport." It is likely the airplane hit a deer on the runway.