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Let’s get this out of the way right now: behemoths are dinosaurs. Dinosaurs have been a feature of D&D at least since the X1 module for BECMI, but as far as I know the new nomenclature is new in Fourth Edition.
While I don’t know for sure why the authors decided on the name change, I can speculate. On Earth we call dinosaurs by Latin/Greek scientific names because we only known them through scientific studies - the creatures themselves became extinct millions of years ago.
The world of D&D, however, is not Earth! They never spoke any Latin or Greek there. And in the setting implied by the Monster Manual, dinosaurs never went extinct. So naturally people who interact with them would come up with common-usage names for them. Another benefit of the alternate nomenclature might be that it frees authors to diverge from currently accepted science a bit. I’ve seen people get strict about it when the “dinosaur” terminology starts getting thrown arround, even in D&D monster books.
The two behemoths presented in the first Monster Manual are described as omnivirous reptiles that live in herds and use their size and ferocity to drive out threats and rivals. Both are Large Natural Beasts, with the “reptile” keyword and Int 2. “Reptile” doesn’t come with any attached mechanics (like “aquatic” does), but I imagine some other monsters might have powers that interact with it. They’re only on the Monster Manual.
This one resembles a real-world ankylosaurus. D&D macetails are ill-tempered and territorial on the wild, but if raised from hatchlings they can be domesticated for use as pack animals. They’re Level 7 Soldiers with 82 HP, and fight with their tails. Their basic attack is a Tail Bludgeon that marks, and they can sweep their tails on a Close burst 1 that targets Reflex, and knocks Medium or smaller targets prone on a hit (in addition to doing damage). The sweep recharges on 4-6.
The specification that the targets must be Medium or smaller is a bit unusual. After all, that covers every possible PC, so it’s theoretically unnecessary. I imagine it’s there in case the PCs have their own domesticated behemoths or other giant allies on their side.
AKA stegosaurus. Also fiercely territorial, with an even fouler mood than the macetail. Even domesticated bloodspikes (used to pull heavy loads or siege engines) are difficult to train and control.
They’re level 9 Brutes with 118 HP, and also fight with their tails. The basic Spiked Tail attack does ongoing damage, and they also have the same Tail Sweep attack as the macetails. In addition to the usual Recharge 4-6 on the sweep, they get to do another one as a free action when first bloodied.
The suggested encounter is level 7 and has a troop of troglodytes (shaman+muscle) using a macetail behemoth as armored support. It’s a all-reptile theme group!
Personally, I’d also add behemoths to human or elven forces. Maybe that merchant caravan you were hired to protect is a train of macetails instead of the mules you’d normally expect, and when brigands attack keeping the beasts from going berserk is as much of a challenge as the fight itself.