A Gaeuko is a demonic spirit from Basque mythology that embodies the night itself, specifically the more malicious side of the night.
A Gaeuko punishes humans who do forbidden work during the hours of night and is especially prone to attacking those who boast that they do not fear the night.
A favored tactic of a Gaueko is to howl a warning via chilling wind "Gaua Gauekoarentzat, eguna egunezkoarentzat ("the night for Gaueko (the one of the night), the day for the one of the day")" - they can manifest as either a cow, a wolf or a variety of monstrous forms.