The longhorn cowfish was first formally described as Ostracion fornasini in 1846 by the Italian zoologist Giovanni Giuseppe Bianconi with its type locality given as Mozambique.

[6] The thornback cowfish is classified within the genus Lactoria, an name that means a "milkcow", a reference to the large spines above the eyes resembling the horns of a cow.

[2][7] The thornback cowfish, like other members of the family Ostraciidae, has hexagonal, plate-like scales which are fused together to form a solid, box-like carapace, from which the eyes, mouth, fins and tail protrude.

[8] The mouth is small and has protruding lips, the upper profile of the snout is straight, the back is somewhat convex, the flanks are concave and the belly rounded, causing the fish to resemble a purse.

[4] The diet consists of small invertebrates, which are exposed by jets of water from the mouth blowing away sand, and picked up by suction from the sea bed.