Pokemon Go often cycles several types of Pokemon in one area, with less common Pokemon that may show up every so often. This is why even in an area plagued with Zubats you can still sometimes find an Eevee wandering through. You can also help influence uncommon Pokemon with the use of Incense or Lure Modules, that pull Pokemon independently from what is being tracked nearby. It is important to note that lures and incense don't always guarantee what appears won't be common, but it can be a better alternative when you are unable to move around.