Lewis & Short

pompī̆lus, i, m., = πομπίλος, a seafish (that follows ships), the pilot-fish, rudder-fish (Gasterosteus ductor, Linn.), Ov. Hal. 101; Plin. 32, 11, 54, § 153; 9, 15, 20, § 51.
This name was applied by some to the nautilus, Plin. 9, 29, 47, § 88.