Shrieking eel

Shrieking eels are carnivorous sea-dwelling creatures that inhabit the waters near Florin.

Characteristics
They produce loud shrieking noises that steadily grow louder when about to feed on their favorite food: human flesh. Buttercup discovers this when she jumps overboard on Vizzini's ship, to escape her kidnappers. The eels circle around her, enjoying the thought of a fresh meal, and move in for the kill. But Fezzik pounds an approaching eel and pulls Buttercup safely back on the ship before the creatures can get a bite.