The story of each animal in Walking With Monsters 2: Reptile's Beginnings

Mesothelae: A prehistoric giant spider the size of a human head, and would be hunting cats if they were alive today. At the end of the episode a Mesothelae spider sees a nest of baby Petrolacosaurus' hatching out of their eggs and escaping. The Mesothelae sneaks upon and kills most of them. The same Mesothelae reappears in the 1st half of the 2nd episode as well. A Petrolacosaurus stumbled on the burrow, and the Mesothelae chases it, eventually killing it. She took her dinner home, but something was wrong. Her burrow was flooding and she had to evacuate. She moved to a different area where she can find another burrow, but her dead dinner is taken by a female Meganeura, an eagle-sized dragonfly. She sees 2 male Proterogyrinus fighting over territory or a female. Then the spider crawls on the camera screen and the scene cuts where she tries to go into other burrows, but other spiders scare her away, because they don't like to share. Mesothelae them comes face to face with an Arthropleura, a giant millipede as long as a car. She then finds another burrow and a Petrolacosaurus comes out of the burrow leaving. She finally gets a new home. She seemed safe when a powerful thunderstorm started, igniting a large forest fire. After the forest fire was over, a Petrolacosaurus headed right into the spider's lair. However, the Petrolacosaurus came out with the dead Mesothelae. It showed that the giant spider wasn't safe at all, since she was shot by lightning and the Petrolacosaurus feeds on her fried carcass. Tim Haines says that the era of giant insects and spiders is coming to an end - as evidenced by the Mesothelae's demise (my mom is scared of spiders). My mom is probably glad that Mesothelae is extinct. But she isn't happy, she's just glad.

Petrolacosaurus: One was seen crawling towards a Mesothelae's burrow and was chased by Mesothelae. The Petrolacosaurus hid in a tree trunk but the Mesothelae broke in from the top and killed it. The spider dragged it away but a Meganeura stole it. Another was seen scurrying past the same Mesothelae. After a massive storm and forest fire during the night, a Petrolacosaurus finds a fried Mesothelae and proceeds to eat it. The show's narrator says that 'the era of giant insects and spiders is coming to an end and Petrolacosaurus begins to evolve into Edaphosaurus, while the scene shifts from Carboniferous to early Permian.

Meganeura: An eagle-sized dragonfly. A Meganerua stole a dead Petrolacosaurus off of a Mesothelae to eat it in a conifer. When a storm and forest fire happened during the night, a group of Meganeura flew around a lake in confusion. The Proterogyrinus in the lake kept trying to eat them in succession but most of the insects got away.

Arthropleura: An Arthropleura chases a Megarachne away from its territory. Then it fights a Proterogyrinus: at first the Arthropleura uses its armored skin to protect itself from getting bitten, but Proterogyrinus pushes the giant creature over, causing the Arthropleura to get impaled by a large tree stump. The Protergyrinus then consumes the carcass by munching on its soft, exposed innards.

Proterogyrinus: A group of Proterogyrinus was basking on the banks of the local lake as a homeless Mesothelae appeared. However, the vociferous amphibians scared off the spider. Shortly after, an Arthropleura came to the lake and a Proterogyrinus confronted it. As the amphibian tried to attack the arthropod, the Arthropleura reared up. The amphibian then pushed it over onto a sharp rock. The impaled Arthropleura's carcass was then eaten. When a storm approached, and as lots of Meganeura scattered above the lake, some Proterogyrinus surged out of the water and ate several of the dragonflies.

Dimetrodon: In the 2nd half of this episode, Dimetrodon played a prominent role, as it was depicted living a lifestyle similar to the modern crocodiles. In particular, the baby Dimetrodon were shown hatching with sails, fully independent. In fact, no Dimetrodon eggs have been found and it's entirely possible that the sail, which would be hard to store in an egg, was either absent or not rigid upon hatching. Furthermore, hatchlings were portrayed sprinting towards trees after hatching in order to escape cannibalistic adults, behaviors based on the modern Komodo Dragon, something that cannot be proven either.

Edaphosaurus: After the Carboniferous reptile Petrolacosaurus evolved into an Edaphosaurus, a herd of them were basking in the sunlight on a hill. It was spring spring, and the Edaphosaurus herd were resting underneath the conifers to shield their sails from the sun. The next day, as the adults rested, the youngsters played and tested their strength. However, over a hill, a female Dimetrodon watched them. As the carnivore edged towards the herd, they noticed and scattered. As the Dimetrodon locked onto a juvenile Edaphosaurus, it attacked and killed the youngster. It then fed on the carrion until a group of larger male Dimetrodon successfully challenged her for the kill.

Seymouria: Seymouria makes a brief appearance in the episode when it prepares to raid the mother Dimetrodon's nest. It attempts the act, but is soon killed by a male Dimetrodon who wished to attack the nest as well.