By our definition, Hallucigenia was not actually an arthropod. It did not have jointed legs or a segmented body covered in a hard exoskeleton.
However scientists have had a difficult time figuring out exactly what it was. In the 1990s, after studying newly discovered fossils, scientists finally identified Hallucigenia as a close relative of a rare modern animal known as an onychophoran. Onychophorans are probably closely related to arthropods.