(the fur of) a kind of animal that lives on land and in water and makes dams across rivers

eager beaver

a person who must work hard and long at unpleasant or uninteresting task

slave, very hard worker

an old horse; a person paid to do hard and uninteresting work as a writer

person who does easy work for money

a person who is the property of another person; a person forced to work very hard for someone else

person who serves, often under duress

a person who finds it difficult to stop working, e.g. a business person, writer, researcher

slave, very hard worker