The term DAM-BUILDING BEAST can be solved by the 6 letter word BEAVER. Beavers are large rodents native to North America and Eurasia that are known for their ability to build dams in rivers and streams. They use branches, mud, and stones to construct these dams, which can be up to 6 feet high and hundreds of feet long. Beavers are also known for their flat, scaly tails, which they use to slap the water as a warning sign to other beavers.