The muskrat is not really a rat, but it is the most common herbivore (plant eater) of Minnesota's wetlands. It is the largest member of our family of native rats and mice (the Cricetidae). The muskrat and its bigger cousin, the beaver, are the only mammals that actually build homes in the water. That is where they spend the winter and raise their young.
For more information, see http://www.dnr.state.mn.us/mammals/muskrat.html