Hydrolycus is a genus of large predatory characins (family Cynodontidae) from South America's major river systems, recorded in the atlas from Brazil, Colombia, Peru, Venezuela, Bolivia, Argentina, Ecuador and Guyana. Members of the genus are widely known by the common name payara, or as vampire fish, for a pair of greatly enlarged fangs on the lower jaw that project upward past the upper jaw when the mouth is closed — an adaptation for seizing and impaling fast, slippery prey fish in open, fast-flowing water. The atlas covers 4 species, and the genus runs large even by predatory-characin standards, from about 12 in up to roughly 37.5 in.
Hydrolycus is prized by anglers across its native range for its size, speed and fighting ability, and species such as Hydrolycus scomberoides and Hydrolycus armatus are the ones most often encountered under the payara name in both sport fishing and, occasionally, specialist predator-fish aquarium keeping — a role that demands a very large tank, strong current, and a diet of appropriately sized live or feeder fish given the animal's strict piscivorous nature and size at maturity. Conservation status looks broadly secure at present: the atlas tally runs three Least Concern to one Not Evaluated, with no species in the genus currently flagged as threatened.