Colonial Americans had an ulterior motive for believing in the flesh-eating mastodon. Unlike the wild continents of Africa and Asia, North America didn't really have any impressively dangerous animals like lions or rhinos. It soon became kind of a joke for the rest of the naturalist world. So the discovery of the mastodon became a source of nationalist pride for the early United States -- a monster so vicious and ferocious that even the rules of basic biology couldn't contain its bloodlust. Thomas Jefferson even assigned Lewis and Clark a secondary mission objective: to find evidence of living mastodons pillaging native villages in the wild uncharted West.
In the meantime, a mastodon skeleton featured in the Philadelphia Museum was altered to look tougher, with its tusks turned downward like a saber-toothed tiger:
Meanwhile, other researchers came up with even more creative theories:
Alexander Anderson, via Common-Place.org
"What big eyes you have."
"THE BETTER TO STAB YOU WITH."
Of course, today's scientists have a better theory about why the mastodon's teeth look like something out of a grinding mill -- they were for grinding. Grinding up branches and tough vegetable matter. As for the elusive claws, they never found any evidence for that either, but hopefully that won't stop them from making a SyFy original movie out of it.