Image courtesy of artisticlicensetokill; see below.
These are the big ones: the two dinosaurs everyone knows (especially since they’re so often pictured dueling each other). If dinosaurs exist in your world, it’s not a question of if the party ranger is going to want one of these as an animal companion; it’s when.
Meanwhile, though, let’s give you some excuses to sic them on your players.
The kasatha matadors of Barsca laugh at the notion of bullfighting. These nimble, four-armed jumpers instead test their skills against enraged triceratopses. Criminals and outsiders who wish an audience with the Saber Chief are often expected to take part as picadors.
A kapre wishes to drive adventurers away from the as-yet-unspoiled Rainbow Falls. The wooden-faced plant entity blows smoke on the adventurers and exposes them to its confounding aura, then drives a tyrannosaurus into their midst to scatter them.
After being awakened, a tyrannosaurus uses its newfound intelligence to ensure a more reliable food supply through banditry, extortion, and mercenary work. Its most recent contract is with Crescent Mining, a company of wereboars currently gutting the jungle east of Portage Lagoon.
—Pathfinder Bestiary 86
My original adventure seed for the tyrannosaurus was, “A tyrannosaurus has been awakened—and taught the law.” If any of you brave readers are up for it, I would love to see your adventure seeds starring T. Rex, Esq.