The genus is classified as part of the Megalograptidae family of eurypterids, a family differentiated from other eurypterids by the possession of two or more pairs of spines per podomere on prosomal appendage IV, a reduction of almost all spines and the large exoskeletons with ovate to triangular scales. It is estimated that Pentecopterus had a length of 1.83 metres (6 ft), turning it into one of the largest eurypterids or arthropods ever discovered. However, Pentecopterus was overtaken by other eurypterids such as Jaekelopterus rhenaniae, the largest known arthropod with 2.5 meters (8.2 ft), or even other genera of its family, such as Megalograptus shideleri, with an estimated length of 2 m (6.2 ft).
A replica of the eurypterid was created for the National Geographic's "Strange Truth" program. The project had the help of James Lamsdell, one of the paleontologists who contributed to the description of Pentecopterus, who provided information and two illustrations. When it was finished, it was taken to the impact site where the original fossils were discovered.
Pentecopterus is among the largest known arthropods at an estimated length of 1.83 metres (6 feet). With this size, it exceeds all genera of the Megalograptidae family, although there are records that Megalograptus shideleri could reach 2 m (6.2 ft), but this is based on two incomplete and fragmented tergites that make this size doubtful.
The large amount of fragmentary specimens recovered of Pentecopterus, including juveniles and exuviae specimens, have allowed an almost complete description of the external morphology. Pentecopterus is diagnosed as a megalograptid retaining a single pair of spines on the third podomere of the third prosomal appendages, an short appendage V with a serrated distal margin of podomeres; prosomal ventral plates widening anteriorly, posterolateral pretelson lacking expansion and xiphos-like shaped telson, with a margin laterally ornamented with scales. The prosomal ventral plate is of Erieopterus-type, that is, it consists of a single plate that covers the anterior and lateral portion of the ventral carapace. The appendages are attached to the soft ventral integument of the plate. The general outline of the carapace was quadrate with an elongate trapezoidal outline.
The most of the fossils of the Winneshiek Shale of Decorah, where the fossils of Pentecopterus have been found, were recovered in 2010 from the upper 4 m section of the from the mentioned site of Iowa. Other samples were collected from blocks eroded during flooding, which are assumed to have been sourced from the uppermost 2–3 m. The total number of fossils found is over 5,000, of which around 6.6% belong to eurypterids. Among the material eurypterid is the holotype of Pentecopterus, SUI 139941, which consists of a prosomal ventral plate and proximal podomeres of prosomal appendage II, in addition to multiple paratypes and additional material, all housed in the University of Iowa Paleontology Repository.