A dragon's flying is (Na) and the tyrannosaurus's ability to swallow is (Ex). As the MM reminds us, (Ex) abilities are allowed to break the laws of physics. Natural abilities, however... there's a wrinkle. Because a dragon's wings can't possibly support him any more than his chest could actually safely combust, or freeze the air, or whatever else each flavor can do.
So D&D doesn't run on the laws of physics; the commoner railgun told us that already. Parts of it - elemental rules in particular - seem to point to a more platonic universal system.
Akasha, also known as The Root, The Origin or (if shifting philosophies)
Taiji, created and continues to define the world. It's basically the source code to the universe. While most spells alter some corner of the universe, a Truenamer can access Akasha to change the nature of a thing, what it is and how it works. The Wish spell (among others) gives brief, unrestricted access to a wizard. The contact is too fast to change much, and certainly too fast to learn information incidental to the goal of the casting.
Templates, Summoning, and Polymorph
Every race has an entry in Akasha. For instance, every orc is essentially an instantiation of the
platonic ideal of Orc, defined by its differences from that ideal than its differences from, say, a human. Another campaign setting could have this defined by one or more deities.

Originally Posted by
Conjuration
Conjurations bring manifestations of objects, creatures, or some form of energy to you (the summoning subschool), actually transport creatures from another plane of existence to your plane (calling), heal (healing), transport creatures or objects over great distances (teleportation), or create objects or effects on the spot (creation).
The summoning subschool doesn't summon an existing creature; we know that already, too. When you summon a bearded devil, you aren't literally grabbing one from hell. Instead, you're shaping aether into the
form of a bearded devil. This is why they're always exactly the same. The prototypical bearded devil doesn't change. The only exception is fiendish and celestial animals, which are slightly altered based on the infusion of elemental evil or good.
Incidentally, this is why summoning a devil is an evil act: It's not that you're teleporting a devil from hell, getting him killed, and helping angels. It's that you're explicitly and tangibly increasing the amount of evil in the world, by creating a new form for evil to inhabit.
Back to Polymorph.
Polymorph allows a wizard (or sorcerer, shaman, Wu Jen, Hexblade...) to briefly touch Akasha. You can't touch it for long enough to take any information back with you; however, you can say, "I want a form that can fight." "I want a form that can fly and hold something." "I want a form that can eat a man." From that, Akasha shapes you - or allows you to shape yourself - into a Green Hag's platonic form, or an Abeil. Of course, if he knows what he's looking for, a mage can ask for a specific form. In fact, if you don't know what you're getting, you may not even know the name of the species you just turned yourself into (although you may get a bonus on any future knowledge checks!)
This neatly explains why you can't turn into a creature with a template. Templated creatures are not stored in Akasha; only plantonic forms of each race exist there.
And yes, I'm aware that this is a mishmash of half-baked philosophical ramblings. I've repurposed some existing concepts for my own worldbuilding and I'm fine with that.