Dinosaur (VGtM)
Open the door, get on the floor, everybody walk the Dinosaur~
Dinosaurs are the largest of mundane beasts, but calling them mundane might be a bit of a stretch considering they are prehistoric beings we only know from fossils. However this is D&D, and people will take their not-explicitly-magical creatures anywhere they can get them. If you want to make the most of these creatures, consider giving a few to your world’s Druid population. If it’s good enough for Eberron, it’s good enough for you.
It’s worth noting that all Dinosaurs now have the Dinosaur tag, just like Cattle having the Cattle tag. What this means going forward is up to any kind of speculation you can muster, but it does leave the door open to things like undead fossil dinosaurs or dinosaur angels. It could even, perhaps, be a possible hint for the return of Saurials. Other than this tag, none of the dinosaurs here had any significant changes.
Brontosaurus
As a gargantuan CR 5 creature, this might be the first, or even only creature of that size your party will ever see. Despite their prodigious bulk, the Bronto is a very simple creature when it comes to combat. It will stomp, to knock prone, then Tail anyone who is knocked prone.
Deinonychus
You know the Raptors from Jurassic Park? These are those, but IRL they are a bit smaller than they are on the big screen, still big enough for medium size though. You can totally rip off all the things they did in the movie and get away with it. But it does afford a hotly debated question: Dino-feathers yea or nay? Personally, I’m for the feathers, if only because you can totally mess with people who don’t expect them. And who knows, you could maybe make an archer that uses dinosaur feathers as their fletching.
Anyway, Deinonychus is brutal in combat. Using its superior mobility, it can pounce on a creature to knock them prone, and get an extra bite attack, for a total of four attacks per round, with advantage on some of them! Considering you get to pick what order the multiattacks go in, save the bite for last, just so you can make sure it will get advantage if the prone happens.
Dimetrodon
OK, so here is the point where I have to point out, much like how D&D treats Mushrooms like a Plant even though they totally aren’t, The Dimetrodon isn’t actually a Dinosaur, even though it has the tag. Aside from that, the only notable thing about this poseur is that they have a swim speed and CR ¼ which means you can summon them for amphibious assaults, if your DM lets you that is. You could also make them into a Beastmaster pet, but that would require playing a Beastmaster using the old rules, and nobody wants to do that.
Hadrosaurus
This duck-billed dinosaur is big enough to ride, which is cool. Do I have anything else to say about them? Honestly not really. They aren’t a complex beast by any measure. The only interesting part of combat for this creature is whatever is riding it.
Quetzalcoatlus
Once again, this isn’t a dinosaur. But it is named after a god, and it's one of the biggest sky beasts ever, which means you use it to fly a few people around, which is cooler than riding around on a hadrosaurus.
In combat, this beast that I have to use copy-paste to even try getting the name right, uses their flyby to joust targets and deal extra damage for their efforts. If they do have a rider, the rider should stick to ranged attacks.
Stegosaurus
Now for a dinosaur you can’t really ride. The Stego has a bunch of plates sticking out of its back, and likes to thwack pesky predators with their mighty Thagomizer. Which is the name for the pointy bits on their tail, in honor of a joke by Gary Larson that is as old as I am. Seriously, it’s a real word, go look it up.
Velociraptor
These would-be birds of prey are about the size of a chicken, but they are also one of the most deadly cr ¼ beasts. Thanks to pact tactics and their tiny size, they can use their multiattack with advantage nearly every turn as it’s almost impossible to block all of them without an actual wall. When a druid casts
Conjure Animals, they are secretly hoping for a pack of these.