#so take this as the fantasy spitballing it is and not scientific truth

[vickysaurus]

Okay, so I am not all knowledgeable about horses, so take all this with a grain of salt because I'm researching them as I type out this post. But the Wikiped tells me knights generally rode medium-weight horses that weighed around 500 kg, although it's possible particularly heavily armoured knights went for heavy-weight horses in the 680-910 kg range instead. So in order to match their ability to carry a fully armoured knight, but also have solid speed and endurance like a horse, I'm inclined to look for animals around that size. Of course, larger dinosaurs might be used in medieval warfare as well, the same way elephants were, and I could imagine some truly incredible howdahs mounted on a sauropod's back that might function as a whole mobile castle. But a Brachiosaurus is not really gonna be doing the job as a knight's mount. We're gonna be looking at more average sized dinosaurs here.

Looking at some videos of them galloping, it seems horses keep their backs surprisingly still compared to many other mammals while running. If you look at a dog running at full speed, for example, you'll see their spines flexing up and down much more than horses do. Which probably means even hobbit knights would be unlikely to be riding dogs into battle. But this is a field where I think dinosaurs should do the job quite well, as the modern ones keep their backs quite still while running:

Of course, horses are quadrupedal, but with dinosaurs we get to pick between bipeds, quadrupeds, and facultative bipeds like hadrosaurs who could do both. A bipedal mount is obviously going to present both knight and mount with some balance issues, since the mount has to deal with balancing a fully armoured knight's weight on just two legs. A bipedal mount is also going to be more prone to 'rearing up' and probably be more unpredictable in its movements in general. And if you do fall off, it's a longer way to the ground. Careful breeding of the mounts, and much training for both knight and mount would probably be needed to ride a bipedal dinosaur. None of this is to say riding a biped would be impossible, but they're probably not the most likely choice. So let's leave Theropods, Pachycephalosaurs, Ornithopods, and the bipedal early Sauropodomorphs to the side for now. I'll come back and write more about how the more daredevil knights might ride them later.

Speed is a concern, so we can probably eliminate Thyreophora (Ankylosaurs and Stegosaurs). Aside from their speed issues, it'd also be tricky to find a comfy spot to ride them from, with their extremely broad backs (Ankylosauria) and armour plates and spikes. If we also eliminate dinosaurs that are too large or too small, I think we are left with only Ceratopsians and the smallest sauropods, such as Europasaurus. A Europasaurus-mounted knight is quite a cool idea, though I think it's probably still too slow, more like riding a cow than a horse.

So we're left with Ceratopsians, and I think these are probably best suited for a knight's needs. They are strong, sturdy creatures, quite capable of great bursts of speed for short durations at least. They've got sturdy backs that would let you put a saddle on them quite stably (though some may have had quills on their backs). Many of them (though probably not Triceratops) lived in herds, meaning humans could probably step in and use their social behaviour to domesticate them, as we've done with many ungulates. Many of them probably engaged in headbutting, which would help them keep their cool when their rider tells them to charge down an enemy. A cavalry charge where the mounts have horns and protective crests of their own to match the knights' lances and armour would no doubt be absolutely terrifying. Most of their crests were built more for display than protection, their skulls having large windows to lighten them, but that could be fixed with some sort of crest armour. I think the ceratopsians might actually quite like having a big chunk of steel on their crests making them look even more impressive, provided the weight is not too much for them.

Ceratopsians would be a much heavier ride than a horse, though. A Torosaurus (or Triceratops) is much more like an elephant than a horse in size. Even the average-sized ceratopsians, like Styracosaurus and Pachyrhinosaurus, would still weigh in at around two tonnes, since they're much bulkier than horses even if they're about the same height. A Yehuecauhceratops could get you some more manouevrability, but if you go smaller than that your mount is probably not gonna be able to carry a whole knight. And there lies the problem: these are bulky creatures, very capable of doing a sprint, but not very good at running around for a long distance. Ceratopsian-mounted knights would take a long time to get from point A to point B, and they're not going to be outmanouevring infantry on the battlefield either.

If you're looking for a dinosaur mount that's more of a long-distance runner, like a horse, you'll unfortunately end up back at the bipeds with all the balance issues they bring. I'm not gonna go through them exhaustively, but I do wanna highlight a few of the more flashy options. A particularly large Utahraptor would be about the right weight, but since they tended to leap onto prey to use their claws and use their feathered arms for balance, you're in for a very bumpy ride. Air-filled bones might make them more fragile than a knight wants too, and as ambush predators they probably exhaust more quickly than horses do. Other mid-sized theropods might make for more suitable, less jumping-prone mounts, particularly if you can find one that's both a social animal (for domestication purposes) and a pursuit predator (for long distance running). Yutyrannus or Nanuqsaurus, perhaps?

As for bipedal herbivores, most Pachycephalosaurians would be far too small, but Pachycephalosaurus might do a good job, provided it went through centuries of breeding for larger and stronger animals as horses were. Like their ceratopsian relatives, they probably wouldn't balk too much at a cavalry charge, but they'd be a lot lighter and more manouevrable. I don't imagine their battering ram skulls would be nearly as useful as their cousins' horns during a charge, but if you're charging right by the enemy like in a joust they'll probably do a good job. The ornithopods come in a wide range of sizes, with many of the most famous ones being too large for a knight's purposes. They were social animals, and quite capable of running around, but since they'd have to be able to go about on both four and two legs to make full use of their mobility, you'd have to use some sort of modified saddle and probably strap yourself in to ride them properly. Many would come with built-in horns for long-distance communication if you can teach them to do it on command too.

So in conclusion, there's no perfect dinosaurian mount for a knight, but there are a number of decent choices, each with their own strengths and weaknesses. Styracosaurus is a good choice for heavy cavalry charging, while Nanuqsaurus or Pachycephalosaurus will do more for a knight who wants manouevrability and speed.

Which dinosaurs would make for the best knightly mounts?

I'm not a dinosaur kid, so you'd probably be better off asking @we-are-avenger and @we-are-scribe who *are*, and can weigh in better than I.