From Muppet Wiki

Herbivore "Herb" the Plant Eater is a red Brontosaurus that appears on Sesame Street.

He debuted as a nameless dino in an Ernie and Bert sketch alongside Bert, the product of Bert's imationation. The dinosaur reappeared in the "Dinosaurs" installment of Elmo's World.

In Season 37, Herb was featured as the main character in episode 4113 (when he gained his name and back-story). It is revealed that Herb is the only dinosaur left except, of course, for his cousin Shirley. While on Sesame Street, he meets many new people and learns something new from each and every one of them. Herb becomes good friends with Slimey.