Beat (or ride) like a rented mule

To abuse something or someone without concern, or compassion for them. To treat property without care because it is not your own.
This is an old expression. A mule is a pack animal, a beast of burden. Since you have no value of the animal you could abuse it by whipping, working, or loading, etc.. "That new guy is slow and lazy, if he doesn't start 'shaping up', I'm going to ride him like a rented mule."