Arthro is an Arthropleura. He was Susan's second creature.
His card, along with the Heat Eruption Move Card, winded up in Mexico. He activated when partying locals kicked his egg into the fire. He then tried to attack a little girl, but was stopped by Panther. He then battled Panther, and won. He then battled Morto, but lost easily. He was later revealed to be a 13 on the Power Scale. Five episodes later, he participated in a battle royal, but was defeated easily with a one-two punch from two Tragedy of the Spheres. The next episode, he participated in another battle royal, but lost again. Two episodes later, he battled a trio of Deinonychus and lost. The next episode, he battled a Precambrian Worm, and was beaten senseless. The next episode, he battled a Smilodon but lost. The next episode, he battled a Thylacosmilus but lost, After the Thylacosmilus' capture, he was removed from the team.
The Heat Eruption Move Card was in his Card Capsule, and was activated along with him.
He has never won a battle under Susan's ownership.