alligators and sharks. They both have that "been around since the dinosaurs" cred. Sharks even more, they're from pre-dino time, have the "wide variety of shapes and sizes" angle working for 'em (cute little dogfish, menacing makos, giant lumbering filter-feeding whale sharks), they've got the "mysterious" thing (how long do great whites live? are sharks really immune from cancer?) Both have several horror movies based on their dangerousness.
