The Chenosaurus was a dinosaur-like mech built within Chen's chair. The chair was held in the Hall of Villainy for a time, with a statue of Chen sitting on the throne. When Chen's spirit possessed the statue, he immediately began playing around with the chair, eventually ejecting a set of robotic legs which later became a complete dinosaur-like mech. Along with Eyezor and Zugu, Chen went to confront Kai and Nya at their parent's blacksmith shop while riding the Chenosaurus. A chase broke out, with Chen relentlessly pursuing them before Nya damaged the Chenosaurus with one of its own missiles.
Chen's chair is full of many mysteries, especially given its vast variety of different and often strange functions. One such function is its ability to contain a transformable mech, which resembles a dinosaur - specifically a Tyrannosaurus rex, especially given its small "arms". The mech is built within the chair, and can transform into its final form after several buttons are pressed. Chen likely had the chair built some time following the Serpentine War and his banishement to his island.
Throughout the Tournament of Elements, Chen sat on his chair but never utilized the Chenosaurus mech. During the Second Serpentine War, Chen left the mech behind on his island. After the war ended and Chen's followers were banished, the chair was at one point claimed and placed in the Ninjago Museum of History.
After the completion of the Hall of Villainy, the chair was placed in the new exhibit, with a replica of Chen sitting in it. On the Day of the Departed, Cole accidentally released the spirits of the Ninjas' old enemies from the Departed Realm, after which they possessed their statues in the Hall of Villainy. Chen possessed his statue, and afterwards began playing around with his chair. While doing so, the Chenosaurus' robotic legs ejected from the bottom. Chen was caught off guard by this, and became frantically as the mech unsteadily ran around the exhibit. Chen managed to gain control of the mech's legs, and after meeting with Yang, he transformed the mech into its complete form and set off to confront Kai and Nya at their father's blacksmith shop.
Chen arrived at Four Weapons Blacksmith riding the Chenosaurus, along with Eyezor and Zugu, who were also resurrected and now rode Condrai Crushers. In his eagerness to have revenge, Chen initially struggled with operating the Chenosaurus, until a giant missile launcher rose from the back of it. Chen and his generals began attacking Kai and Nya, who set off on their Raider Cycles. After Kai's cycle crashed, Chen tried firing a missile launcher at him, but Nya saved him in time.
Nya and Kai rode through Ignacia, with Eyezor and Zugu in close pursuit before the two villains were outsmarted and defeated once more. Chen fired multiple missiles at his adversaries throughout the chase, but was unable to hit them. After losing his generals, Chen fired a single missile at Kai and Nya - however, Nya used her element to repel the missile and send it flying back towards Chen. The missile struck the Chenosaurus, destroying Chen's statue and sending his spirit back to the Departed Realm.
After suffering minor damage, the Chenosaurus, along with the chair that contains it, were likely returned to the Hall of Villainy following the event.