The celethi is found in some medieval encyclopedias. It is a marine monster that lays more eggs than any other. The young grow to resemble their parents very quickly. It sleeps so soundly that it can be caught with a hand.

Th celethi is a misunderstanding by Thomas of Cantimpré of Aristotle's account of the selache, which is the name of a genus of fish, not a particular fish. Albertus Magnus corrected Thomas's account, and says "celeti is a genus of animals containing many species under it."