Merchandise-Driven: One of the biggest examples of this — the reason for the production of the animated series was so that Tyco could launch a Dino-Riders toy line. Oddly, in some markets there was no animated series, or there was just a one-off special. Some markets (France in any case) had a comic book.
Ptero Soarer: The Dino-Riders had Quetzalcoatlus (Yungstar's mount of choice), Pterodactylus, and several small Rhamphorhynchus which came with the Brontosaurus set. The Rulons had Pteranodon, and as late addition also a Quetzalcoatlus. A leopard-patterned one.
Raptor Attack: The Dino-Riders and the Rulons have Deinonychus among their forces. Until the introduction of a Rulon Quetzalcoatlus at a quite late point within the toy line's run, it was therefore the only species that was ridden by both factions. (If you only count the toys, and not the series or the comics.)
Remember the New Guy: Happened a lot with both Dino-Riders and Rulons as the toy line expanded to include more characters. Most notable example; the Commandos, who didn't appear at all in early episodes but were featured prominently in the last few and starred in the Ice Age special.
They had dinosaurs like Tyrannosaurus rex and Diplodocus coexisting, which were separated by millions of years' time.
The line of toys included Dimetrodon, which A) was not a dinosaur and B) was long extinct before the first dinosaurs came into being.
Also, Woolly mammoths, saber-toothed cats, and giant sloths are thrown into the mix. These were collected from the Ice Age and brought back via time travel. Yeah.
Brontosaurus. Enough said.
Made worse by the fact that the "Brontosaurus" was at the centre of a two-part storyline that had the Dino-Riders and Rulons battling over it because it was "the largest dinosaur in the world." A great idea in concept, and to an eight-year old it was an absolutely epic story, but considering the Dino-Riders made casual use of a Diplodocus, which was bigger than Brontosaurus/Apatosaurus to begin with, they really dropped the ball. Should've made it a Brachiosaurus.
As far as doing research goes, Diplodocus was lengthier than Apatasaurus/Brontosaurus but nowhere near as heavy.
The greatest irony is that at the time they were released, the toys for the franchise were considered some of the best, most realistic depictions of dinosaurs available anywhere. Not so much these days... They were fully articulated, unlike most dinosaur toys, so that was cool.
Oddly, the distinctive-looking T. rex figure appears to have gained another very weird life as the inspiration for Rex in Toy Story!
Snake People: Rasp, Sidewinder and whole bunch of several unnamed snakemen.
Spotlight-Stealing Squad: As the show's short run went on, episodes seemed to centre more and more around the GI Joe-style Commando team.