The term 'marsh rodent' refers to two species of aquatic rodents that inhabit wetlands and marshes. The first is the muskrat (Ondatra zibethicus), which is a medium-sized rodent with a thick, brown fur coat and a long, scaly tail. It is found in North America and parts of Europe and Asia. The second is the water rat (Hydromys chrysogaster), which is a large, semi-aquatic rodent native to Australia and New Guinea. Both species are important for their role in maintaining wetland ecosystems, as they help to control vegetation growth and provide food for other animals.