"That description tallies with fossil remains of a family of extinct ground sloths known as the mylodontids," he said. "These animals had dermal ossicles -- bony armor plating embedded in the skin."

Reports have long circulated about a "mapinguari," a legendary and terrifying manlike creature of the vast Amazon rain forest, but when scientists took an interest at all, Dr. Oren said, they tended to guess that the animal, if it existed, might be some kind of primate.

"But when I began hearing accounts of a creature with shaggy red hair, backward-turned feet and a monkeylike face, I realized that witnesses might have encountered a ground sloth, closely related to extinct giant sloths known only from their fossils."

Three families of sloths are known, and only two genuses, both of them tree dwellers, are known to have survived to the present day, Dr. Oren said. The common three-toed sloth is a lethargic creature that seems to live in slow motion, and is considered a family member of the megatheriids, known mostly from fossils.

The much rarer modern two-toed sloth, a surviving member of the megalonychid family, is less "slothful," Dr. Oren said, and can move swiftly and forcefully if threatened. The third family of sloths, the extinct, ground-dwelling mylodontids, grew to the size of large bears and were apparently very active.

The mapinguari, as Indians call the supposed forest creature, is apparently smaller than fossil members of the family, standing only about six feet tall when walking on its hind feet, but weighing some 500 pounds, and with jaws and feet powerful enough to rip palm trees apart. The creature is said to subsist largely on palm hearts and other vegetable delicacies of the rain forest. Agreement on Appearance

The mapinguari is also described as having a thunderous voice that can sound quite human, and that has deceived human visitors to its habitat into thinking that another human was nearby.