You know, Glaurung could be just as scary because he has no backstory. He pretty much drops into the story out of nowhere as a fire-breathing dragon.
I propose that Morgoth makes upgrades to him as things progress. Instead, he has the fire-breathing first since we hear about that power first with a taste of the budding manipulation/silver tongue he has, then tactical skills like commanding troops in battle and being a good general which we'll see in the Dagor Bragollach and Nirnaeth Arnoediad, then in his final form in the second half of The Children of Hurin, he's developed the mind powers that send Turin on a wild goose chase to Dor-Lomin and wipe Nienor's memory. And along the way he gets progressively bigger (though his belly remains unprotected since Azaghal has to deal him a severe enough injury he has to flee the Nirnaeth, and Turin stabs him in the belly; presumably he is able to kill Glaurung because he has a longer weapon than Azaghal).