Genghis Rex is the tyrannical leader of the Tyrannos and the owner of a very serious and rigorous yet wise and flexible personality.
He is a brutal, ruthless, oppressive, and ill-tempered evolved Tyrannosaurus as well as Allo's evil counterpart. Despite this, he is respectable and admirable Allo at times. Also, his relationship with the Dinosaucers is not as bitter as that of sworn or archenemies or even enemies at all but rather as an extremely disputed and, sometimes, even somewhat friendly rivalry.
He loves to be flattered by being referred to high-sounding names such as "Bossasaur" and "Your Scaliness", yet he hates being called "Chiefasaur". He calls his minions offenses like "Idiot-tops" and "Tail-For-Brains" when they screw up or when he cannot admit his failures and discounts his wrath upon his minions.
He has an equally evil sister named Princess Dei. Genghis' and her sister's parents were once the rulers of Reptilon before the Dinosorceror and Dinosorceress took over.
He has a blind, extremely deep, head over heels passion for Teryx, but she loves Ichy, so she always denies his advances. Despite such denial and Teryx's own admission to not have feelings for Rex, she occasionally does seem to display pity and concern toward him. However, he does not seems to be offended by her refusal and eventually seems to come in terms with the reality with the help of Ankylo, though he never truly gives up and expects for better and easier times to come, at least when it comes to ruling the Earth.
Although he may be seem feral and mindless at first glance, Genghis Rex is actually very manipulative and intelligent, especially when it comes to scheming and planning ingenious moves against the Dinosaucers, although they all ultimately fail either by his comrades' clumsiness or due to the Dinosaucer's well-thought-out actions and luck. He is also shown to be highly comprehensive as he do not make fun or mock his comrades and fully grasps the meaning of the events when someone betrays him, which happened to him by the likes of Quackpot, Plesio and Terrible Dactyl due to their matters of conscience and embedded instincts. He even consents to be impossible to be loved reciprocally and ultimately complies with this, though this was arduous, difficult, and burdensome - perhaps even traumatizing - by which one can see a fragile and soft side of Rex rarely, if ever, shown under any other circumstances.
- He was voiced by Dan Hennessey.