When Disneyland first opened, they suddenly realized all the food on the premises attracted mice and other rodents. When human exterminators proved unable to contain the infestation, Disneyland turned to feral cats.
Originally the feral cats were unwanted but when they proved better at controlling the rodent population than professional exterminators, Disneyland officially allowed the cats to stay. In fact, they even have a dedicated, official web page explaining more about the Disneyland cats.
Of course the irony is that Mickey Mouse is a rodent and helped create Disneyland, but feral cats running freely in the park are kept specifically to keep out other rodents. If you ever see a feral cat taking down Mickey or Minnie Mouse, then you’ll know that the cats have gone a little too far.