The Arms and Equipment Guide list several of them as possible mounts, and gives several examples on how to use them, like a Dwarven pike squad on the back of a triceratops.
Duplicated in Eberron, where Halflings use some of the smaller dinosaurs as mounts.
They can be found in 4th Edition as Behemoths and Drakes, and given Cadillacs and Dinosaurs style names—Macetail, Bloodspike, et cetera—apparently because while dinosaurs are cool, sciencyish names aren't fantasyish enough.
In 3rd and 3.5, you could play as dinosaur inspired player races called saurials.
They made a miniature of a Fiendish T-Rex. As if a T-rex wasn't enough, they slapped the Fiendish template on it.
Exalted, being a setting whose content is decided entirely by Rule of Cool and Ninja Pirate Zombie Robot, has gigantic "Tyrant Lizards" that look remarkably like T. rex'es. Presumably due to Tyrannosaurus being Latin for "Tyrant Lizard". It also has the Dragon Kings, which are roughly human-sized sentient dinosaurs who wear armor, can learn to breathe fire, and have enormous crystal clubs with laser blades to hit you with if you piss them off.
Warhammer Fantasy's Lizardmen are a dinosaur race, their basic troops being "Saurus" and them using "Stegadons" and "Carnosaurs" as mounts and war engines. There are also "Cold Ones," Velociraptor-like things used by the Lizardmen and Dark Elves for cavalry. The beasts eat everything, so the Dark Elves cover themselves in poison to fool the lizards.
The Cold Ones are mentioned (literally or metaphorically) to be dragon-kin...the ones the Dark Elves use also are subterranean, can live in the cold climate of the Dark Elves' territory, and, IIRC, sweat poisonous goo, which might make them more of a freaky lizard-horse than a dinosaur.
A lot of Death Worlds in Warhammer 40,000 are covered in dinosaur-esque creatures, and the Tyranids themselves are also somewhat dinosaur-y, at least in terms of teeth - the Termagaunts, especially, look like an odd cross between Armadillos, Zerg Hydralisks and Velociraptors.
It is worth noting that the Zerg Hydralisks (from Starcraft}) are probably inspired by Termagaunts (not the other way around). Meanwhile, Termagaunts look like a cross between Alien Xenomorphs and bugs from Starship Troopers.
Eldar Exodites dwell on lush, primeval worlds inhabited by saurian "dragons," which they joust on with laser lances.
Possibly explained in older literature. Death Worlds were likely worlds seeded by the mysterious Old Ones in the time before the Eldar started to record their history. Since the Lizardmen of Warhammer Fantasy were servants of the Old Ones, it's likely that worlds they settled would have dinosaur-like creatures 'imported' to those worlds.
And they use huge beasts called Knarlocs, which are basically T Rex meets terror bird. Which is even better.
Werewolf: The Apocalypse has the Mokole, which are weredinosaurs. Technically, they're supposed to be werecrocodiles, but since they serve as Gaia's Memory, their battle form can include body parts of any reptile from Earth's history.
Magic: The Gathering has a few, including Old Fogey from the Unhinged parody set, an allosaurus rider, a pygmy allosaurus, several "raptors", and a bunch of other made up dinosaurs.
In Pathfinder, druids can take dinosaurs as animal companions, as can clerics with the Saurian domain. Cavaliers can also ride dinosaurs if they choose a beastrider archetype, and there's even a special druid archetype called "Saurian Shaman" which is basically a mechanical version of this, They do anything related to dinosaurs better than the same non-dinosaur-related actions (such as summoning, wild empathy, and wild shape).