Cornered Beast is an upgrade for Guapo, Juan's crocodile Amigo. Guapo takes less damage once his health drops below half, which helps him survive long fights.
Once Guapo is badly hurt, incoming damage is reduced. He lasts longer at low health, which gives his own healing time to work.
Keep Guapo in fights until he has recovered 3,000 health in combat. The upgrade then unlocks.
Body Regulation heals him during combat, so between them he spends less time at low health and more time fighting.