D&D 5E Unearthed Arcana: Giants survey is up

[Azzy]

The psi warrior is a pale imitation of its 3.5 forebear. Unfortunately. And I agree with the doctor—the mystic was trying to do everything at once. I think that's the reason it failed.

 

[Paul Farquhar]

Because it can just as easily be a giant snake, or a saber-toothed tiger, or a wooly rhinoceros? This isn't a stretch, the flavor text is pretty clear that "dinosaur" is just one option.

Or a bear, or a horse, or a camel or an axe-beak. It differs from other (post-Tasha's) pets only in that it is bigger and tougher.

 

[Kurotowa]

Or a bear, or a horse, or a camel or an axe-beak. It differs from other (post-Tasha's) pets only in that it is bigger and tougher.

Indeed, "big and tough" seems to be one of the two broad restrictions for it. You don't get a terrier, you get a dire wolf. The other one is that it's a land based behemoth, not a creature of the skies or seas. Which is probably a mix of wanted to keep the creature on-theme and the usual balance concerns.