pilot-fish

noun, plural (especially collectively) pi·lot·fish, (especially referring to two or more kinds or species) pi·lot·fish·es.

1. a small, marine fish, Naucrates ductor, often swimming with sharks.
2. any of various other fishes having similar habits.

n.1630s, from pilot (n.) + fish (n.). So called because they were thought to lead sharks to prey.