In "Paw Patrol," the character Rex first appeared in Season 9 in an episode titled "Rubble's Big Movie," where he is introduced as a dinosaur pup. Rex becomes an ally of the Paw Patrol and is particularly helpful in tasks that involve his special abilities related to dinosaurs.
Later, he joins the Paw Patrol in the "Big Truck Pups" special, which features the team upgrading their equipment and working with larger vehicles. This transition into a more integral role within the team allows for more dynamic storytelling and new adventures. Rex's character adds a unique element to the Paw Patrol, and his joining the team in a new context speaks to the show's theme of teamwork and collaboration among friends with different skills.
If you have specific episodes or storylines in mind, feel free to ask!