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An animal that is active at twilight, whether at dusk or at dawn, is said to be crepuscular. Used in the same way as nocturnal or diurnal. Horses, peculiarly enough, are crepuscular by nature.

Can also be used to describe events that occur in the twilight hours, or more metephorically, events pertaining to the dawn of man or to the end of a human life (cf: "twilight years"), though seldom vice-versa.