Dragon turtles have their origin in Chinese mythology, combining the power of two of the four "auspicious beasts" to bring together their positive aspects. As such, while they don't make direct appearances in mythic tales, they are apparently common in artwork and statuary due to their supposed positive effect on feng shui. Having said which, the version that appears in D&D, while doubtless inspired by the mythic creature, also has features in common with the European conception of dragons and certainly doesn't seem much like a feng shui ornament.
1E
The 1E version of the dragon turtle, so far as we can tell, has much in common with traditional depictions of the creature. It has a turtle-like shell and a long, thick neck that probably doesn't retract inside (as, indeed, is the case for some real-world turtles). The head is clearly draconic, with a snout and fangs rather than the toothless beak of turtles, and a pair of feelers or decorative tufts on the forehead. Significantly, it also has the prominent dorsal crest seen on many D&D dragons. We can't see the limbs in the illustration, but we're told that they have claws - something true of most real-world turtles although, as it happens, not of the deep sea sort, which have flippers.