Rats... OK, let's try a different approach...

Is it Aerodactyl?

From ages past, I’m the defender.
Aerodactyl lived 5 million years ago.

My strong attacks will make you shiver.
I think that shivering implies fear, not cold; I'd be scared if a stone-skinned pterodactyl-dragon-bat-monster was swooping down at me at 150mph, wouldn't you?

I own the greatest move of all,
I think Aerodactyl could learn Explosion in RBY; nowadays, it can still learn Hyper Beam...

Though to get it I have to stall.
...But not until it reaches Level 50.

Don’t crack under the tough stress
Aerodactyl has the Pressure Ability; stress = pressure.

Or you’ll fall to the battling best!
Aerodactyl was once known as 'king of the skies.' It likely was the top dog of prehistoric Pokemon.