The term DAM-BUILDING RODENT is referring to the BEAVER, a 6 letter mammal. The BEAVER is a semi-aquatic rodent native to North America and Eurasia. It is known for its ability to construct dams, canals, and lodges using tree branches, mud, and rocks. Beavers are the second-largest rodent in the world, after the capybara. They are an important part of the ecosystem, as their dams create wetlands that provide habitat for other species.