>>57409207The biggest mistake canon works often commit with legendaries is functionally treating them as as nothing more than a mix between a plot device and a damsel in distress. They really only exist to be the thing the antagonists need to accomplish their evil deeds, and their role in the plot is to be saved by the protagonists to stop this. As a result, legendaries often have no agency or ability to influence the plot as characters, and this is especially ridiculous considering their own lore and status should mean that they're the most well equipped to defend themselves and enact their will out of any Pokemon or character in the setting. This becomes truly absurd in the later anime films, where villains are choking out mythicals with their bare hands and the capital G God (whose mere awakening shifted the realms of reality itself) is nearly killed by a medieval dude with hardening liquid and a bunch of Electric-types, just so they can fit into the role of things the protagonists need to save.
The other big issue I have with canon is legendaries and mythicals becoming too prominent and in your face; there's no longer a real sense of mystery or discovery with them anymore, and they've become inexplicably well known and categorized even in-universe, which kind of defeats the point of these creatures being considered the stuff of myth and legend. Fanfics tend to repeat the first mistake wholesale and get even worse with the second, where knowledge of legendaries and what they all do and look like is somehow common among everyone. Then there's the pantheon trope where legendaries are either some organized force or at a bare minimum all know and regularly communicate with each other, which makes the world a lot smaller and comes across as a fundamental misunderstanding of the kind of deities and legends these beings are inspired from and meant to represent. I have other issues with fanfics' treatment of legendaries of course, but this post is long enough as it is.