The Bird That Devours Men
Of course until Louis Joliet and Father Jacques Marquette fought with it it was just an NDN legend.

Be sure to check the links at the end of the story.
Many moons ago, there existed a birdlike creature of such great size, he could easily carry off a full grown deer in his talons. His taste, however, was for human flesh. Hundreds of warriors attempted to destroy the Piasa, but failed. Whole villages were destroyed and fear spread throughout the Illini tribe. Ouatoga, a chief whose fame extended even beyond the Great Lakes, separated himself from his tribe, fasted in solitude for the space of a whole moon, and prayed to the Great Spirit to protect his people from the Piasa.
On the last night of his fast, the Great Spirit appeared to Ouatoga in a dream and directed him to select 20 warriors, arm them each with a bow and poisoned arrow, and conceal them in a designated spot. Another warrior was to stand in an open view, as a victim for the Piasa.
When the chief awoke in the morning, he told the tribe of his dream. The warriors were quickly selected and placed in ambush. Ouatoga offered himself as the victim. Placing himself in open view, he soon saw the Piasa perched on the bluff eyeing his prey. Ouatoga began to chant the death song of a warrior. The Piasa took to the air and swooped down upon the chief. The Piasa had just reached his victim when every bow was sprung and every arrow sent sailing into the body of the beast. The Piasa uttered a fearful scream that echoed down the river, and died. Ouatoga was safe, and the tribe saved.