Mercury: They used to have the Roman messenger god with the winged hat as their logo. Then sometime in the '80s, they went with a futuristic styled M (dubbed the "Flying M").

Oldsmobile: It's a rocket. Represents the old Rocket engines.

Chevrolet: Two stories about that. The first one is that the founder saw the pattern on the wallpaper of a hotel he was stying in. Second story is about how it represented the Swiss flag (Louis Chevrolet's heritage).