From Creatures Wikia
Big Fish is the evolved form of a Tiny Blue Fish. Every winter, (usually) one Tiny Blue Fish moves to the right shallow end of the pond and changes into a large mottled brown creature that resembles a mud skipper or catfish. This large fish can move about on both land and sea, and will eat any fruit, seeds or food it can find. At the end of the winter season, the Large Fish returns to the pond if able, and lays Tiny Blue Fish eggs. After laying eggs, the large fish dies, leaving behind a fish skeleton.
While scary looking, the Big Fish is harmless to Norns, if they play with it or try to eat it, the Big Fish will reduce their drive to reproduce temporarily.