A group extinct (purists will dispute this, and refer to the fact that, according to cladistics, birds are surviving dinosaurs) reptiles (again purists will dispute this, because grouping the dinosaurs together with snakes, lizards, and even turtles, is stretching the definition of 'reptile' to the extreme - they will argue that calling dinosaurs a group of archosaurians would be much more accurate). Emerged in the late Triassic and became the dominant land animals during the Jurassic and Cretaceous periods. About 65 million years ago, they became extinct in a mass extinction event (you are free to blame Thande for this). Creationists claim that none of this is true, as dinosaurs lived alongside of humans and breathed fire, silly!
Dinosaurs have become quite prominent in popular culture over the past few decades (Jurassic Park, et al), which resulted in countless nerds all over the world becoming fascinated with them.
Recently, dinosaurs have invaded Doctor What's apartment, but they were successfully driven off by his cat Sphinx…
Dinosaurs's are featured prominently in Doctor What's Extinction Event. They are also found sporadically in ISOT scenarios. In particular allosaurs and tyrannosaurs are popular for these purposes.