The Trochus, “wheel”, or Rota is a huge sea-monster known to swim close to shore in large groups. Schools (pods?) of these have been seen off Athos and Sigeum.
A trochus is fortunately timid, despite having a crest and spines of great size that show above the water. It revolves and contracts and dives deep, uncoiling and rolling and returning to the surface.
The wheel-like resemblance suggests a jellyfish or ray, but the size and behavior makes it clear that the trochus is a whale surfacing and diving.
Aelian, trans. Scholfield, A. F. (1959) On the Characteristics of Animals, vol. III. Harvard University Press, Cambridge, Massachusetts.