Arthropleura armata

Arthropleura was a giant relative of centipedes and millipedes that lived in Great Britain and North America during the Carboniferous period. It is the largest known land invertebrate of all time, and grew so large because of the high concentration of oxygen in Carboniferous Earth's atmosphere.

Despite it's relationship to centipedes it was actually omnivorous, or possibly even herbivorous, but could still have had venomous bites to protect itself from the few large predators that lived in the Carboniferous.

Paleobiology
Arthropleura was a giant millipede, it grew 1-8.5 feet (0.3–2.6 metre) in length and was covered in hard armored plating. Despite it's menacing appearance it was an omnivore or possibly an Herbivore.

Errors

 * Arthropleura was not believed to be venomous by any means, since it presumably had no enemies and ate dead plant matter.