There are two types of dragon lusii, pseudodragons and true dragons. Pseudodragon is basically a catch-all term for any vaguely-reptilian creature that is big and scary and will bite your face off. True dragons are basically the same, but typically have some extraordinary psionic ability, like Pyralspite’s eye beams/explosions(? in that one flash it’s a beam, but on Jake’s island the same type of dragon just makes an explosion. Could just be that the power manifests differently from individual to individual, or can be applied differently.) This psionic aptitude also makes them more susceptible to animal communion.
i imagine it’s kind of like when an animal is called a dragon on earth (komodo, bearded, etc). it obviously is not a dragon, but is named so for its superficially reminiscent reptilian qualities.
i reckon alternia has a bunch of flying reptiles, some maybe related to dragons, some not, which just get called dragons in common language for the same reason.
it probably helps that the comic implies, gently, that, like rainbow drinkers, dragons are considered myth (”In case it wasn’t clear, dragons are real.” is a direct parallel with ”In case it wasn’t clear, magic isn’t real[.]”, which refers to the long-running gag where things that clearly are/aren’t real are erroneously denied as being actually/not actually real); it makes perfect sense for a troll to see an animal that isn’t a dragon and go “that’s a dragon!” because they actually don’t know what a real dragon is like, exactly.
I also think that given their airborn nature (and apparent above-ground breeding habits), it’s likely that true dragons (eudrakonia?) aren’t actually part of the lusus population; the pyrope line is just sort of a fluke that was allowed to happen because dragons are smart enough to choose to be custodians if they want to.
Yeah, Terezi’s case is weird because her lusus hasn’t even hatched yet! Reading over the wiki entry for it made me realize how odd it was, and how much it deviates from the typical lusus/troll adoption system. I could definitely see them as having developed independent of lusii. And then, pseudodragons could be trollkind’s efforts to recreate the mythical depiction of dragons through breeding programs, the same way lusii like Aurthour are the attempts to reproduce musclebeast-like traits. Trolls WOULD be the kind of people who want their lusii to look like a Super Cool Mythical Animal What Shoots Fire From Its Eyes.
Which would also account for a much wider range of forms a pseudodragon could take, the same way depictions of dragons across cultures varies wildly (lindworms vs Chinese loong vs the typical four leg, winged western dragons)