The sea-mouse is found in some medieval encyclopedias. It is a marine fish that comes out on land to lay its eggs in a trench, which it then covers; after thirty days it returns to take its offspring to the water. It swims in front of whales to guide them past dangerous shallow areas.

In modern taxonomy the sea-mouse is Aphrodita aculeata, a marine worm found in oceans around Europe. Because its body is covered hair-like structures (setae) it resembles a mouse when it it is washed up on shore. It does not breed on land or guide whales.