Brine Dragons are an obscure species of aquatic dragon native to the Dungeons & Dragons setting of Dragonlance, although the name has also been used in Pathfinder for the Plane of Water-dwelling branch of its Primal Dragon family.
Although brine dragons claim to be descendants of Black Dragons, their appearance is so massively different that the truth is impossible to verify based on that alone. Fundamentally, a brine dragon looks like a cross between a plesiosaur and a dragon, with a draconic head and a long, serpentine tail ending in a horizontal fluke attached to the snake-necked, four-flippered, "shell-less turtle-like" body of a plesiosaur. Lacking claws, their teeth have overdeveloped to compensate, which has the side-effect of causing them to look like they are grinning sadistically all the time. They are wingless, and also flightless, which you wouldn't think needs to be said explicitly, but these are dragons and thus levitation could still be on the table. Their hide is rough and mottled, covered in irregularly shaped, ill-fitting scales and myriad ridges and crags. This adds a bit of credence to the theory that brine dragons were an experiment in making an aquatic dragon by Takhisis, similar to the Sea Dragon. Despite their ugly looks and lack of magical abilities, brine dragons are superlative natural predators, able to move through the water with incredible stealth and being extremely hard to ambush.
The reason why brine dragons are called such is because of their strong biological association with salt. Brine dragons love salt. Their bodies are invariably dotted with huge clumps of the stuff, some of which are so old that they are discolored by the dragon’s bodily secretions and are no longer able to be dissolved in the water. They inhabit the oceans, but sometimes come inland to inhabit estuaries, salt marshes, and other briny waters along the coastlines. Even their breath weapon is a lethal salt-based gout of corrosively alkaline gas.
In AD&D, it's mentioned that Tinker Gnomes are fixated on the idea of getting to see what will happen if a brine dragon and a black dragon aim their breath weapons at each other. They have tried to make this happen for decades, but since black dragons despise their self-proclaimed descendants with a passion, all that's happened is that a lot of gnomes have ended up dead. Which isn't entirely a loss, one must concede...
For the curious, the official answer to the gnomes' question in AD&D is that the two breath weapons will neutralize each other, as the black dragon's breath is a gout of sulfuric acid. The result is that the sprays will transmute into boiling hot but chemically pure water, as well as spawning several pounds worth of sulfur.
Aside from this love of salt, brine dragons will eat anything, and notable as one of the few living beings that consider the undead to be an enjoyable foodstuff. The one thing that they won't eat is the flesh of a black dragon, due to a combination of respect and awareness that acidic meat in an alkaline-saturated gullet is a recipe for biological disaster.
Personality-wise, these dragons are best described as violent, aquatic anarchists with nihilistic tendencies. They are, by nature, capricious and unpredictable to the extreme - it's extremely difficult to predict what a brine dragon will do in any situation. You can't even rely on them to be hostile, because a brine dragon is as likely to try and talk an attacker out of harming them whilst withholding their own force as they are to attack without provocation. They have no system of rulership, no leader, no society of any sort. The Sea Witch Sagarassi has long since given up trying to recruit any brine dragons for her causes, since the beasts are just as likely to breathe on her troops as on any enemy. Even the other evil races of Krynn regard brine dragons as an embodiment of chaos.
The beasts make their lairs out of coral and rock formations, using their caustic breath to create a convenient cave; this forms the sole point of fixed interest in the brine dragon's territory, which can range from 100 yards to 10 miles in diameter depending on how the dragon feels that day. Their impulsive nature means their hoards tend to be smaller and more eclectic than you'd expect, since a brine dragon only grabs treasure when it feels like it.
Fortunately for Krynn, brine dragons are quite rare, because they are terrible parents; their usual whimsical nature shines through in breeding, so a clutch of brine dragon eggs and hatchlings may be reared by one parent (usually the mother), both parents, or neither parent, as they both get bored and abandon them. And even if they do stick around to look after the eggs, they may get bored one day and cannibalize their own offspring.