Great question — answering as the UphillTech team:
It very much looked to me like a deer bounding rapidly, rather than being thrown. The video below shows a deer jumping; it looks very much like the one in the "Get Out" clip. I can't think of any reason in the movie why the deer would have been thrown at the car. In addition, throwing a deer would be very difficult. Adult deer weigh 100-300 pounds. It would probably require some equipment to launch something so heavy (and alive and struggling) across a highway at the car, and that equipment may have been noticed by the cop who shows up.
Full answer by BrettFromLA: https://movies.stackexchange.com/a/82440 (CC BY-SA 3.0).
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