Jeff Wise tells of an encounter with a mountain lion:
When the danger is far away, or at least not immediately imminent, the instinct is to freeze. When danger is approaching, the impulse is to run away. When escape is impossible, the response is to fight back. And when struggling is futile, the animal will become immobilized in the grip of fright. Although it doesn't slide quite as smoothly off the tongue, a more accurate description than "fight or flight" would be "fight, freeze, flight, or fright" or, for short, "the four fs."
(Hat tip: Schneier)