Boxfish, also called trunkfish, or cowfish, any of a small group of shallow-water marine fishes of the family Ostraciontidae (or Ostraciidae), distinguished by a hard, boxlike, protective carapace covering most of the body.

The alternative name cowfish refers to the hornlike projections on the heads of some species. The members of the family, found along the bottom in warm and tropical seas throughout the world, are considered good to eat and are often dried as curios.

Except for the eyes, the low-set mouth, and the fins and tail, boxfishes are encased in the rigid carapace. This covering consists of fused plates, and in cross section, depending on the species of fish, it takes the shape of a rough triangle, square, or pentagon. 

Boxfishes are often very attractively coloured. They are small, the largest growing to about 50 cm (20 inches) long. When captured and handled, boxfishes exude a toxic substance that can kill other fishes kept with them.

Related to the boxfishes are the keeled boxfishes of the family Aracanidae. These fishes also have a carapace, but there is a keel along the underside and openings behind the dorsal and anal fins. The members of this group are found from Japan to Australia.

Ostraciidae, from the Greek ostracum, meaning "shell", the box, trunk or cowfishes are found all over the tropical Atlantic, Indian and Pacific oceans. Fourteen genera with about thirty three species. These may be the most characteristic of fishes with their bodies encased in a bony carapace.

The family is also notable for some members possession of "ostracitoxin" a toxic discharge substance of some trunkfishes. If/when sufficiently disturbed whole tanks, and recirculating systems of livestock have been wiped out. Though this is rare, the cautions listed elsewhere in this piece on netting, acclimation, and tankmates should be heeded if you intend to try a trunk/boxfish.

The genus Lactoria, especially Lactoria cornuta should be dealt with carefully; I have witnessed three poisoning "episodes" with this species, one from a dead specimen in a filter, another from a harassed individual, the third from the careless introduction of shipping water into a display system.

Lactoria and members of the genus Lactophrys are commonly called Cowfishes for their "horns", a conspicuous pair on the head and another doing rear-guard near the anal fin. The Smooth Trunkfish, Lactophrys triqueter is the most frequently offered member of the genus, with the Scrawled (Lactophrys quadricornis) and Honeycomb (Lactophrys polygonius) cowfishes occasionally available. All three hale from the western tropical Atlantic and grow to at least eleven inches in length.