Rex

Rex is a plastic Tyrannosaurus Rex with a lot of anxiety from an inferiority complex ("I don't think I could take that kind of rejection!"). He is insecure about his lack of ferociousness. Rex's worst fear (after Sid) is that Andy will gain another, scarier dinosaur.

Toy Story
In Toy Story, Rex states that he was made by a subsidiary of Mattel (coincidentally, real-life Rex toys are now made by Hasbro).

Rex tries to avoid confronting Woody when he is accused of trying to jealously kill Buzz Lightyear, but slowly and reluctantly turns against him under peer pressure, quickly experiencing guilt later on. He is known to have a queasy stomach and actually barfed when he saw Buzz's dismembered arm.

By the end of the film, he seems to have gained confidence throughout the movies expressing hope that Andy will receive a leaf-eating dinosaur for Christmas that Rex can play with and intimidate.

Toy Story 2
It is revealed in the sequel that he is a video game fan and is very frustrated that he cannot beat the "Buzz Lightyear: Attack on Zurg" video game. After Woody was stolen by Al McWhiggin, the owner of the Tri-County branch of Al's Toy Barn, he attempts to help by getting the address to the FBI, although he gets Mr. Potato Head frustrated due to his accidentally wrecking his crime scene simulation. He goes on a mission led by Buzz to rescue Woody after Woody is stolen by Al McWhiggin of Al's Toy Barn. After the toys enter Al's Toy Barn, Rex excitedly finds a "Buzz Lightyear" video game manual, only to lose it later, but is still able to give Buzz (the toys take an "alternate" Buzz instead) some secrets he has acquired from the manual. Rex is considered to be the heaviest of Andy's toys when he loses his grip on the new Buzz's utility line and pushes the toys to the bottom, causing the new Buzz's strength to give out. Later, "alternate" Buzz and the toys use Rex as a battering ram to break into Al's apartment. After the toys leave the apartment, they encounter a Zurg action figure, whom "alternate" Buzz duels with Rex watching. As Zurg is about to finish the "alternate" Buzz off, Rex accidentally knocks Zurg off the elevator with his tail; however, in the process, he finally overcomes his frustration of not being able to defeat Zurg previously.

Toy Story 3
In the third film, Rex eagerly awaits the opportunity to finally get played with by children again. In the opening scene of the film, he reprises one of his roles from the first film while Andy is playing with him as the giant dinosaur that eats forcefield dogs. Years later, Andy is 17 years old, and Rex, like the other toys, wishes so much to be played with that he even gets excited when Andy picks him up. The toys are accidentally almost thrown away and believing that they are just junk to Andy now, decide to sneak into Andy's mom's donation to Sunnyside Daycare box. Once at Sunnyside, they are welcomed by the other toys that reside there, led by Lotso, the Strawberry scented bear (as remarked by Rex), who show that Sunnyside is a paradise for toys where you will be played with by generations and generations of kids. The toys there take Rex and the others to the Caterpillar Room, telling them that they will soon be played with by children. When the kids are returning to the room, Rex eagerly waits in front of the door, excited that he's finally going to get some play time. However, when the kids enter, it is discovered that they are very little children who have no regard for treating a toy right, even breaking off Rex's tail. When the toys try to complain to Lotso, it is revealed that they were sent to the Caterpillar Room to be beat on by the toddlers on purpose and that you have to earn joining the Butterfly Room. Lotso even reprograms Buzz so that he's back to being a senseless astronaut who keeps Rex and the toys in shelved baskets (which function as prison cells). To distract Buzz, Rex and Hamm have a little fight, and when Buzz attempts to break off the fight, Jessie calls to him and entraps Buzz in a plastic storage bin, on which Rex and Hamm stand on to prevent his escape. In order to reset Buzz, Woody has Rex insert his finger into a tiny hole below the switch, which causes Buzz to shut down for a brief second, then spring back to life in his Spanish deluded version. After a series of attempts and planning, the toys finally escape Sunnyside through the trash chute but are confronted by Lotso, who had "broken" Chatter into informing him of the escape plan, along with several of his henchmen. Woody, having been informed of Lotso's past, brings up the subject of Daisy, causing Big Baby (and by extent the other Sunnyside toys) to turn against him. When the toys return home, they clean themselves of the dirt and grime that had made its way onto them during their escape. Finally, Rex is given to Bonnie by Andy. During the end credits, he is seen becoming close friends with Trixie the Triceratops, often playing computer games with her. Finally, he and Trixie are seen side by side as the toys enjoy watching Buzz and Jessie perform a paso doble.

Trivia

 * In Toy Story on Ice, Rex has a wife named Mrs. Rex.
 * Rex quotes a few of George McFly's lines from Back to the Future: "I don't like confrontations" and "I don't think I could take that kind of rejection."
 * Rex makes a cameo in the outtakes for Monsters, Inc. where he tries out for the role of Ted, the gigantic monster whom Mike and Sulley say hi to on the way to work.
 * You can hear Rex's voice from Wallace Shawn in The Incredibles as Mr. Huph.
 * Rex appears at the beginning of "WALL•E" for a brief second when WALL•E is opening the door of his truck. Since it's a close-up it can be very easy to spot.
 * According to the official movie magazine for Toy Story 3, Andrew Stanton is credited for the final design of Rex's face in the original film.
 * While inside Al's Toy Barn in Toy Story 2 Rex appears in a rearview mirror (viewed by Mr. Potato Head) as he tries to catch up with the car, parodying a scene from Jurassic Park where a Tyrannosaurus Rex appears in a rear-view mirror when chasing a crew in a jeep (although for entirely different reasons as the T-Rex in the film was trying to eat the car, while Rex was attempting to get back in the car). Coincidentally, Rex's mature roar is a duplicate of the T-Rex in Jurassic Park.