The way I think about it is, "Will the animal due damage to my unit."  

We have long known that diseases, especially viruses, can pop up from animal sources.

Perhaps, perhaps not, but apartments and mobile homes are different animals.

Pacing animals can do a bunch of damage.

You can ban smokers, pets (but not ESA/Service animals), multi-generational families (with two rooms, there isn't space for grandma and the kids to move in).

Animals? 

Hi Michael,Obviously they've chosen to keep the dog and move somewhere where they won't be thrown out.Some people will get rid of an animal, some will move instead.Good Luck!

Originally posted by @John Underwood:The best advice I have received on a emotional support animal is to make your tenant go to petscreening.com to validate the real need for the animal.I also use petscreening.com for all my animals.

You don't want the tenant to be training their animal while living in your apartment unit.

Unfortunately this was an ESA animal, but considering they left it and did not know how often they needed to change the litter I have my doubts about it being needed (even if they did provide documentation).