RIO BRANCO, Brazil — Perhaps it is nothing more than a legend, as skeptics say. Or maybe it is real, as those who claim to have seen it avow. But the mere mention of the mapinguary, the giant slothlike monster of the Amazon, is enough to send shivers down the spines of almost all who dwell in the world’s largest rain forest.

The folklore here is full of tales of encounters with the creature, and nearly every Indian tribe in the Amazon, including those that have had no contact with one another, have a word for the mapinguary (pronounced ma-ping-wahr-EE). The name is usually translated as “the roaring animal” or “the fetid beast.”

So widespread and so consistent are such accounts that in recent years a few scientists have organized expeditions to try to find the creature. They have not succeeded, but at least one says he can explain the beast and its origins.

“It is quite clear to me that the legend of the mapinguary is based on human contact with the last of the ground sloths,” thousands of years ago, said David Oren, a former director of research at the Goeldi Institute in Belém, at the mouth of the Amazon River. “We know that extinct species can survive as legends for hundreds of years. But whether such an animal still exists or not is another question, one we can’t answer yet.”