Dragon Turtles are species of monster in the worlds of Dungeons & Dragons. Distant relatives of the dragon family, like the wyvern or the dragonne, these giant sea-dwelling monsters resemble enormous sea turtles with clawed flippers, long draconic tails, and a vaguely dragon-like head, although the art for them has changed a lot over the editions. Dragon Turtles are to the seas what the Chromatic Dragons are to the land; avaricious bullying brutes who use their immense strength and their superheated steam breath weapon to terrorize shipping lanes, sinking anyone who can't bribe them with treasure for their hoards.
The dragon turtle was based on a mythical Chinese creature of a much more benevolent nature, a spirit symbolizing courage, determination, fertility, longevity, power, success, and support. D&D promptly gave this a nod by creating the "Oriental Sea Dragon", an Oriental Dragon equivalent to the Dragon Turtle.
Dragon turtles have been around in D&D for a long time. Their very first appearance was as an illustration on page 40 of the 1974 rule booklet Dungeons & Dragons, Volume 2: Monsters & Treasure, and soon afterwards they gained their own profile as a monster on page 34 of Dungeons & Dragons, Volume 3: The Underworld & Wilderness Adventures. Dragon turtles made the jump to both Advanced D&D (AD&D 1e Monster Manual, AD&D 2e's Monstrous Compendium Volume 1 & Monstrous Manual) and to Basic D&D (Expert Set, Companion Rules Set, Rules Cyclopedia, the modules The War Rafts of Kron and Duchy of Ten). They were present in both versions of the Monster Manual for 3e, and whilst they were absent in 4e for some strange reason, they made a triumphant return in 5th edition, once more taking pride of place in the Monster Manual. As OGl content, they appeared in the first Pathfinder bestiary. The only notable non-system wide change is they lost Snatch for Awesome Blow.