Seems like there are/were dinosaur mobs in both Everquest and WoW. It wouldn't surprise me if there were some in Dungeons and Dragons Online as well, since it's based on 3.5, which had all sorts of dinosaurs in the Monster Manual. I should know; I once played a ranger with a deinonychus as a mount. Otherwise known as the actual name of the dino that was mis-named "velociraptor" in Jurrassic Park (real velociraptors were the size of a medium dog, not larger than a grown man.)
As for something based entirely around it, though? I'm not sure that exists.
There is, however, an old DOS game called Designasaurus 2 that I always thought seemed like a dino-based RPG, or at least a dino action adventure, but I never could get it working as a kid. The version I had was on a single 3 1/4" floppy, when apparently the game had been programmed for multiple 5" floppies, and it kept asking for a second disk that literally did not exist. The label even said "disk 1/1," meaning it was the only disk.
There's a review on <link=http://www.hotud.org/component/content/article/47-education/22797>hotu.