Rex was a disciple of Pope and a member of the ape soldiers in Caesar's Ape Colony. Pope took Rex and his soldiers to scout the San Francisco Zoo, where Rex followed Pope when he heard wailing in Cora and Fifer's direction and investigated. After Pope finds Fifer and Cora under attack by primal apes, Rex joins the rest of the troops to defend Cora and Fifer and make their escape.
After Pope kills the primal alpha and enslaves all the primal apes at the zoo, Rex guards the entrance with another gorilla on horseback. When the gorilla is knocked off his horse by Fifer as he escapes, Rex thrusts his spear into Fifer's back. Cora holds off Rex as Fifer makes his escape, but Rex beats and captures Cora as a result and Fifer later dies of his wound after he reaches Caesar and warns him about Pope's plans. Rex interrogates Cora, who claims not to know where Fifer is going and reports to Pope about this, who suspects Cora is lying. At the winter section of the zoo, Rex witnesses Pope interrogate Cora, who remains silent, and later give the order to have a polar bear devour her. At the entrance, Rex asks Pope if Caesar will come and Pope says Caesar will send either Maurice or Koba instead of him. Right before Pope is about to punish a primal ape for touching him, Rex informs Pope his scouting party looking for Fifer led by Bon is back.
In the center of the zoo, Rex listens to Pope give a speech about how they are strong and Rex interrupts to show him Koba has arrived with his own army to stop Pope. Rex joins the rest of Pope's apes in hooting after Pope declares he makes apes strong. After Koba attacks Pope in anger, Rex joins the battle fighting for Pope. When Koba knocks Pope down, Rex climbs to the top of a cage and jumps to ambush Koba while Pope distracts him. Rex manages to strike Koba in the shoulder with his spear causing him to drop his spear and watches Pope taunt Koba while Rex holds the leash to one of Pope's primal ape slave. Rex however fails to see the primal ape rip his leash off and viciously attack Pope just as he is about to kill Koba. Before Rex can help Pope he is stabbed in the leg with a spear by Koba. Koba then declares an ape who betrays Caesar must die and Rex's last act was to yell "NO!" before Koba plunges the spear into Rex's chest, killing him.
More to come...
Rex seems completely devoted to Pope and follows each of his orders without question, even if it mean killing his fellow apes. Rex also seems very protective of Pope as he jumps to Pope's defense whenever he is danger.
- Rex was the secondary antagonist of the ape storyline in the Dawn comic books.
- Rex is exclusive to the continuity of the Dawn of the Planet of the Apes comic books published by BOOM! Studios.
- Rex is the first ape killed by Koba.
- Dawn of the Planet of the Apes #3
- Dawn of the Planet of the Apes #4
- Dawn of the Planet of the Apes #5
- Dawn of the Planet of the Apes #6