Unless you are certain that the smell is a dead animal, it is most likely mildew. I don't know any brand names, but there are solutions that you can purchase to flush out the air intake for the climate control system. Perhaps a dealership can advise.
Blocked drain hose? Start the engine and let the car idle for ten minutes with the A/C set on the coolest setting and verify the presence of clear water on the ground under the cowl area of the car. If none is present, check the interior carpeting for even a hint of moisture under the dashboard area. If the carpet is wet, the A/C condensation drain hose is either detached or blocked. If the carpet is dry and you spot water on the ground than the hose is either clear or only partially blocked.
Regarding my suggestion on the first paragraph: I have had luck ridding my A/C system of slight mildew odors by going to a self-service (wand-type) car wash and spraying soapy water into the cowl-mounted air intake for the climate control system. A temporary cure, but inexpensive.