I was having a discussion with several people on whether or not a lightning dragon can be biologically plausible, without any magic involved. We came to two main possibilities and I'd love to see feedback on them:
1- If it was a conventional flying dragon, it would be the conduit rather than the source of electricity. While gliding among the clouds, its large wings would gather the static electricity in the water molecules and somehow store it in its body. Then the dragon directs the stored electricity into a bolt aimed at the target. In such scenario it would only be able to shoot lightning in ideal weather conditions and while flying. I, however, really lack the knowledge of how electricity works exactly or affects an animal in this scenario, and whether this makes much sense. One of the people in the discussion suggested the dragon could have an organ specialized for storing said electricity, but that really doesn't sound how organs work to me.
2- The most plausible option I could come up with is if the dragon was heavily inspired on electric eels, and in this case it wouldn't really shoot lightning nor be able to fly at all. The issue with electric eels whenever people try to use them as explanation for electric/lightning powers is that, well, it's often overlooked that only this portion of their bodies is the actual fish, the rest is literally all specialized muscle and organs necessary to create such high voltage of electricity. If we were to apply that to a large dragon its body would look nothing like a conventional, flying dragon. It would require a proportionally huge portion of its anatomy to be specialized musculature and organs, and also possibly have a sluggish behavior, given its metabolism will be focused on charging up. My friend and I even doodled rough sketches of what this kind of concept could look like, you can see our attempts here and here. We aren't certain of how electricity would affect the teeth(I've heard that enamel teeth really doesn't go well with it) and feathers, and in such scenario the animal's attack would be based on delivering a discharge via biting or slamming its muscular tail on the target.