What is the most common wild animal in the US?

That depends on what you call an animal.

Short Answer: Microtus pennsylvanicus

The common field mouse (Meadow Vole) is the most common mammal in North America.

Long Answer:

  1. Insects are animals and by far the most numerous. (But I assume you're excluding them.)
  2. What about birds, though? The common sparrow is pretty numerous, even in cities.
  3. There are probably as many rats living in New York City as people. Perhaps more. There are probably more rats in Redhook, Brooklyn than squirrels in Yellowstone Park.
  4. And let's not talk about fish.
  5. For mammals, rodents are the winners with the mouse most numerous.
  6. For large mammals, deer.
  7. Predators, foxes or coyote.
  8. Birds, probably the starling or house sparrow.