Piranhas

Legendary reputation
There are various myths about piranhas such as how they can dilacerate a human body or cattle in seconds. These myths refer specifically to Pygocentrus nattereri, the red-bellied piranha. A recurrent myth is that they can be attracted by blood and are exclusive carnivores. A Brazilian myth called "piranha cattle" states that they sweep the rivers at high speed and attack the first of the cattle entering the water allowing the rest of the group to traverse the river. These myths were dismissed through research by Helder Queiroz and Anne Magurran and published on Biology Letters. Nevertheless, a study in Suriname found that piranhas may occasionally attack humans, particularly when water levels are low. Attacks on humans are usually reported around docks where fish are frequently gutted, and entrails are commonly thrown into the water.
• Theodore Roosevelt
When American President Theodore Roosevelt visited Brazil, Brazilian fishermen set up an incident in which a cow was consumed by piranhas. The locals blocked off part of an Amazon tributary with nets and dumped hordes of starving piranhas into it when the adventure-loving Roosevelt explored the region on a hunting trip. A cow was tossed into the river, setting off a wild feeding frenzy that quickly reduced the carcass to bones—the "instant skeleton" now associated with piranhas.