In Breton lore, Yann-an-oed (or Yannig) is a sea monster that swims off the coast in the hours of darkness to find prey. It makes the cry of an owl. Those who are tempted to replay to it straight away give away their location to the Yann-an-oed who will sneak up and devour them. During the hours of daylight, it lives in the deep sea.
John and Caitlin Matthews, The Element Encyclopaedia of Magical Creatures, 2013, Yann-an-oed, p 530