A dog
From memory he was a guard dog that worked at a prison and was owned by a prison guard while Mickey was in prison, for unknown reasons. However Mickey escaped, and Pluto went with him.
If you think that it's strange for Mickey to be in prison, it's not. Old Mickey was quite the pyscho, and even tried to shoot Donald Duck with a gun because their musical concert was turning to trout and Donald was going to walk out.