The Deep Dragon first appeared (possibly) in the Monstrous Compendium Appendix: Forgotten Realms for Advanced Dungeons & Dragons 2nd Edition, and from here was reprinted into the Monstrous Manual. Due to this, very, very little lore was provided to give the Deep Dragons any real detailing. All that players are told is that they are cunning, patient, and their two greatest priorities are survival and the thrill of the hunt... which also somehow translates into them loving to explore. They amass hoards of treasure they scatter in trap and spell-guarded caches. Deep Dragons live deep and far in the Underdark; their claws are capable of tearing effortlessly through stone, and combined with their innate ability to manipulate earthen environments through the spell-like abilities of Rock to Mud, Telekinesis, Move Earth, Passwall and Disintegrate as they age, they can go just about anywhere. They have a distinct taste for Underdark seafood - clams, blindfish, Kuo-Toa and aboleths above all else - but will eat anything in sight to survive. They work with the drow, but hate the illithids. For pragmatic reasons they avoid conflict with other dragons.
These early deep dragons are one of the few non-Metallic Dragons of AD&D with the ability to shapeshift, being able to transform into a dragon, a giant winged worm or snake, a human or a drow. Dragon Magazine would notes that they produce their own distinct breed of Half-Dragon with drow partners. Their breath weapon is a cone of flesh-corroding gas.
They subsequently returned for Dungeons & Dragons 3rd Edition in "Monsters of Faerun", which had no new lore for them.