Theory: The reason why Nu Udra is particularly vulnerable to Dragon Element, and why the Rakna-Kadki armor set has notably less Dragon Resistance (though still some) is due to their preferred residence in volcanic locales. I think that the adaptations common for volcanic monsters make them particularly vulnerable to the Dragon Element.
I thought of this after looking at other non-Wyvern Volcanic Monsters (as it is pretty standard that Wyverns, especially Flying Wyverns, are vulnerable to Dragon). Additional damage from Dragon Element is quite common, though they of course often take no additional damage from Fire and usually have a weakness to Water. This is particularly for monsters that are endemic to volcanoes, like Ajarakan and Agnaktor.
Whatever Dragon Element is, whether in the blood of Elders or weaponized, it is clearly costly to the animal which produces it. In MHWorld, we see how the Elder's Recess is so strongly favored by the Elders Kushala and Teostra, which we already know consume minerals frequently. The abundance of exposed minerals is likely optimal for the upkeep of their specialized blood and their nomadic lifestyles may be a behavior to continue searching for more mineral-rich locales. This isn't even getting into the extraordinary abilities Elders often have.
This may also be why Stygian Zinogre is frequently found in volcanic regions, while the typical Zinogre is a more temperate montane animal. Dragonfell berries WERE frequently found in volcanic areas (though MHWilds now has them literally everywhere except the Basin, which is weird) and are likely a kind of fruit formed by endemic metallophytic or hyperaccumulating plants in said locales. If baby Basarios or Uragaan are consuming any of such plants, then Dragonfell bushes/trees/etc. may be a decent chunk of their premature diet. Stygian, lacking the easy Gargwa and Arzuros it is likely used to, may have to start taking the heavily biomineralized Rhenoplos (a herbivores which may also consume Dragonfell berries when given the opportunity).
Similarly, this could be said for why Magnamalo can be found in such locales. The Shrine Ruins seem like a similarly ideal habitat for Zinogre as the Misty Peaks are, but the fact that Magnamalo can be found often in volcanic regions is likely due to the mineral-cost of its Hellfire. Further, Scorned Magnamalo is able to produce Dragon Element. It is likely forced out of the Shrine Ruins for less productive habitats (due to it being an outcast with broken horns and damaged armor) and the fact that so many Elders end up appearing around the Citadel may well have drawn them there. Notably, regular Magnamalo cannot be encountered in the Citadel, so the Citadel, while relatively nearby to Kamura, is probably beyond the typical range of the Kamura population.
For Ebony Odogaron, the Rotten Vale likely has just enough dead Elders, or generally just a ton of minerals within corpses, so the species can readily maintain enough Dragon Element. The more nomadic Ebony Odogaron likely takes a more proactive approach to finding corpses, perhaps with a preference for scavenging Elder Dragons as well.
For Deviljho, the species may come into conflict with Elders so notably because of the nutritional stress they face. Deviljho is starving, as a rule, and the expenditure of Dragon Element it engages in to survive infections as a rampant scavenger and frequent combatant probably are causing it nutritional stress as well.
Now, this does not explain by Ebony Odogaron, Stygian Zinogre, and Magnamalo are encountered in the more Arctic/Frigid locales as well as volcanic ones. For that, I have another theory: Dragon Element has some thermoregulatory properties. Elder Dragon Blood in particular may be another enabler for elders throughout the series to be so nomadic, despite many lacking obvious specializations for extreme temperatures.
Because of this, when Dragon Element is used against monsters that rely on it for thermoregulation, it not only causes a kind of allergic reaction in their immune system, but also messes with their ability to regulate their internal body temperature.
Even without vertebrate blood, Nu Udra may well depend on some convergent ability to thermoregulate that still causes it great harm when disturbed. It may also be more vulnerable to Dragon Element than arthropods due to how Cephalopods also often have closed circulatory systems. Rakna-Kadaki still retaining an open circulatory system may be a big deal in how the effects of Dragon Element cannot induce the potential anaphylactic effects that it does in other monsters.
Notably, neither Ceadeus nor Nakarkos give Elder Dragon Blood in MH3U and MHGU respectively. The two fully aquatic Elder Dragons just don't give it, which is a bit strange to me. Further, Ceadeus is still weakest to Dragon, while Nakarkos is also very vulnerable, if that helps at all.
Wait a minute.....is Ceadeus the only monsters in your Hexapterygian group that doesn't give Elder Dragon Blood? Perhaps it lost the ancestral feature due to being secondarily aquatic?
That’s actually kinda weird ceadeus doesn’t give elder blood. Maybe that was just overlooked? I know nakarkos doesn’t because it’s a cuttlefish and Fatalis doesn’t because it’s special or something.
The idea that dragon element also has thermoregulatory properties, and that when disrupted interferes with a monster’s ability to maintain stable temperatures in extreme environments on top of having an immune response makes a ton of sense. And that bug monsters are immune to dragon because they’re biologically different enough from vertebrates, but cephalopods coincidentally are slightly more similar and are vulnerable.
Fuck, that’s really good. I might actually have to add this to my phylogenetic tree docs next time I update them.
I’m forwarding this to UHC