The entire concept, pitch and idea for The Dinosaur Lords is "medieval knights ride into battle on dinosaurs".
In The Dresden Files, Harry Dresden lives this trope at The Climax of Dead Beat. Necromancy practiced on human remains is forbidden on pain of death for wizards, but Harry needs a source of necromantic energy to avoid getting killed by the Negative Space Wedgie the bad guys are trying to claim for awesome godlike power. It's been previously established that animal remains leave thinner "psychic footprints" than human remains, but that the longer a thing's dead, the more powerful it is when it's finally raised. Harry puts two and two and two together and necromantically animates "Sue," the mostly-complete Tyrannosaurus rex skeleton at the Field Museum of Natural History. That's right, Zombie T. rex! Which he then proceeds to ride across Chicago to fight an army of ghosts and zombies.
In Codex Alera: turns out that aside from picking up hapless romans, Zergexpies, and elves, Alara also grabbed a few giant ground sloths and terror birds in it's day. They're bond creatures with the elves.
Eric Garcia's series of noirish mysteries (starting with Anonymous Rex), which feature dinosaurs living in modern-day America, disguised as humans.
The recent science fiction story "Rejiggering the Thingamajig", published in Analog, stars an alien sentient T. Rex Buddihst. And it's awesome.
In the epic children's book Dinosaur vs. Bedtime, the titular dinosaur engages in ferocious battle with all manner of opponents. When faced with the likes of a pile of leaves, a bowl of spaghetti or talking grownups, DINOSAUR WINS! ROAR! But Dinosaur still has to face his greatest challenge: Dinosaur vs. Bedtime! Bedtime wins.
Steve Alten's Meg novels do this. The first scene of the first book, has a Tyrannosaurus get eaten by a Megalodon (giant presumed relative to the Great White Shark) whilst attempting to chase down Hadrosaurs in the shallow sea. Nevermind that Megalodon evolved almost 50 million years after the Dinosaurs went extinct.
Aleksandar Žiljak's (Croatian SF writer) story "Opal eyes" ("Oči boje opala") features a Wild west setting in a world where dinosaurs roam the plains. Also, the main character is a lesbian vampire gunslinger.
The Icemark Chronicles's third book has the enemy Basilea use 'Tri-Horns' as cavalry. Three guesses as to what they are.
Far-Seer is basically the story of Galileo if he was a T-Rex. It is indeed as awesome as it sounds.
The world in which the fantasy novel Ten-Ghost takes place has domesticated dinosaurs everywhere, and the less that domesticated variety prowling the wilderness, for seemingly no reason other than to have them.
The first part of Dinoverse might not count; a young nerd puts a dinosaur bone into something that turned out to be a time machine and ends up in the Cretaceous. But then each of the other times the machine works, even without a dinosaur bone involved, it took people to other points in the age of dinosaurs - even when it was made clear that there were dozens of groups who'd been sent to times millions of years apart, no one ended up before or after that era. Maybe this was just to fulfill the title?
Humorously inverted in The Science of Discworld, in which the UU wizards conclude that dinosaurs are terribly dull creatures that nobody would ever take an interest in.
In Robert A. Heinlein's Between Planets, the intelligent life on Venus is Draco Veneris Wilsonii and they are commonly referred to as "dragons" but are a six legged multi-ton saurians with manipulator tentacles from their necks rather than arms and hands. They also have a dozen eyes on eyestalks and speak in musical whistles.
In the third book of the Samurai Cat series, Tomokato and Shiro face a Nazi Germany that has successfully cloned dinosaurs, including the ambitious Ubersaurus Rex; none survive the book. In the next book, Tomokato visits an alternate dystopian future universe where he was never born, and Rex is The Starscream to the Malevolent God-Emperor of the Universe; Tomokato kills this one as well. Finally, in the last book of the series, "Samurai Cat Goes to Hell", Rex is among the generals in Hell's special security police scheming to overthrow Satan and conquer Hell. In all of these, there are numerous smaller tyrannosaurs and velociraptors...all in Nazi uniforms, no less.
After visiting the Cretaceous era in Chrono Hustle #6, Jack and Mary get a triceratops that they use to ride around in different eras.