The Hyenas were a group of women that were recruited by Jennifer Goines from the J. D. Peoples Mental Hospital to destroy the Army of the 12 Monkeys. She called them Hyenas because (Jennifer claimed) the hyena is the natural predator of the monkey.
Ultimately the Hyenas were sort of a proto-version of the survivor group that Jennifer founded after the plague, the Daughters.
In "Hyena", the Hyenas had destroyed laboratories belonging to either the Army of the 12 Monkeys or Markridge Group. However, they were lured into a building where a bomb had been set by Pallid Man and all the Hyenas except Jennifer were killed.