Horses and Donkeys

One day, a farmer decides to put his five donkeys and five horses together in the same field to save space. He moves the animals into the empty field and leaves them with plenty of grass and hay to eat and water to drink.

One year later, he decides to move the animals back into separate fields; but after moving the horses and the donkeys out, he finds that as well as himself, there is still one animal left in the field. During the year, no animal moved in or out of the field, so what is this extra animal and where has it come from?