[2][3] Agnostus is from the Ancient Greek άγνωστος (ágnostos) "unknown", and reflects the fact that early paleontologists did not know what type of animal with which they were confronted.

The species epithet has this origin: Like all Agnostida, Agnostus is diminutive and the headshield (or cephalon) and tailshield (or pygidium) are around the same size (or isopygous) and outline.

When Agnostina are enrolled, a tiny opening between the cephalon and the thorax becomes visible (the cephalothoracic aperture) that uniquely distinguishes this group from the Eodiscina.

The subgenus Agnostus (Homagnostus) has a broadly rounded termination of the axis extending nearly to posterior border furrow.

[4] Agnostid appendages are known only from exceptionally preserved specimens of Agnostus pisiformis from the Upper Cambrian Orsten deposits of Sweden (Müller & Walossek 1987) [5] and the appendages of meraspid developmental stages of A. pisiformis (½-1 mm total body length) are the best preserved of any trilobite found so far.

The most forward pair of "legs" has a small coxa, a seven-podomere exopodite carrying long, bristle-like setae and gill-like extensions, and no endopodite.

Magnus von Bromell in 1729 described disarticulated heads and tails in matrix as "minimorum vermiculorum vaginipennium" ("small beetle-like worms").