Scorpions are invertebrate animals that belong either in the arachnid family, or in their very own order of the arthropods (opinions vary). They are distinguished from other arachnids by possessing grasping, crayfish-like pincers and having the last segment of their bodies extended into a stinger, complete with a pair of venomous glands.
Brontoscorpio ("thunder scorpion") was one of the first scorpions that had evolved. In Walking with Monsters ep. 1, it was shown as an amphibious animal, much larger than the modern scorpions (who generally do not surpass the 20 cm. mark) and possessing 2 pairs of pincers, as opposed to the 1 used by the modern species. It also appeared to be lacking the modern scorpions body 'hairs' with which they sense their surroundings in order to prevent being ambushed (as Brontoscorpio was ambushed by Pterygotus). It also showed having not only gills, but also the 'book lungs', still used by modern scorpions and spiders with which it absorbed oxygen
A more modern version of the scorpion was also shown in that episode of WWM; in fact, it was live-acted by one of the modern scorpion species. Being much smaller than Brontoscorpio has been, this scorpion proved to be an easy meal for the Hynerpeton (an amphibian-like tetrapod vertebrate), and was probably shown to contrast the Devonian with the Silurian, to show how the early invertebrates had shrunk since their glory days.