Animals get hit by cars all the time, sometimes they are awake enough to crawl into someone's yard before dying. Animals will hide under neath porches and patios and sometimes will crawl into holes in foundation and die. They can get very stinky very fast! Different types of decomposer will have different smells. Always call a professional to remove because of sickness or disease.  

Seal up holes in foundations. Fence around the bottom of your porch, so there are not gaps where the animals can crawl under to die. This will help keep them from having babies and doing other things as well.