Hm, the rot dragon should slowly rot itself. That would be pretty awesome. If it's possible for something's skin to secrete two poisons, then it should do that. If not, then its skin should just secrete the weaker poison.
Here are a couple more dragons:
Spiny Dragon (D): These dragons have spikes on their limbs and tails, making them more dangerous than other dragons in melee. Their scales are also tougher than normal dragon scales, though not as strong as the metal dragons. When it's possible, the spikes will be able to break off and get stuck in their enemies (like porcupine spikes) and regrow.
Magma Dragon (D): Red dragons are fire resistant, but not magma resistant. Magma dragons have skin made of bauxite and spit fire and globs of magma. Due to their tough skin, they move significantly slower than most other dragons.
Wind Dragon (D): They can't breathe anything, but wind dragons are quite fast and can fly. Their scales are just as strong as normal dragon scales, but weigh next to nothing, making them useful for armor.
Blue Dragon (D): They're fast and amphibious, and move even faster in water. Their breath mimics the slowing power of water by stunning all in its path, though anything accustomed to water is completely immune.
As a fan of naga, I'm going to mod them in two. Except they'll be half snake, half man, half dragon. If it's possible to control how much of something a creature breathes out, resulting in breath attacks that only go two or three tiles out from the creature, then I'll use some of those. If the breath types are only one huge forward blast and one small undirected burst (Toady mentioned the undirected burst), then the naga will only have claws and fangs.
All naga will be different castes of the same species, unlike dragons.
Red Naga (N): This naga has a higher-than-average body temperature and its scales are more heat-resistant. It can inject a literally burning venom into its victims.
White Naga (N): White naga are cold-resistant, and their venom freezes you.
Green Naga (N): They're faster and lighter than regular naga, but are also weaker. They prefer to attack from a range and have even adapted to spit small globs of stun-poison.
Iron Naga (N): They are a full size larger than most naga, and have sharp claws that tear through flesh with ease. Their iron scales cover their arms and heads as well as their tails, offering more protection. They move more slowly than most naga, and don't learn things as easily.
Rot Naga (N): Rot naga live short lives. They have some of the most deadly venom, eating away at their enemies' flesh, but they slowly rot themselves with it. They're immune to pain but also only live 10-20 years, while other naga live five times that long.
Wind Naga (N): These naga are faster and weaker than green naga, and if Toady ever fixes flight for civilized creatures and makes flight more detailed, they'll be able to fly short distances.
Blue Naga (N): Blue Naga are amphibious and move faster in water, making them quite useful for taking down carp and other sea creatures.
Gold Naga (N): These naga are the rarest kind. They're exalted and almost worshiped by the other naga.
Also, a new species of dwarf. Beardwarves (☺). Half, dwarf, half bear, these guys are larger than humans and take out anything with their claws. They can live in both forests and mountains, and devour the corpses of their enemies. They are more primitive than other races and learn slowly. In addition, all their weapons are crude, so they usually stick to their claws or trade for weapons. If the next release fixes non-default weapons, the only weapon they'll make with much proficiency is battle-claws.