Hello there and welcome to PC! :D
The weather was stormy.
I have issues with this first sentence. Although there's technically nothing wrong with it, all the description that comes after it now seems redundant. I would take it out since I'd much rather the description show me it's stormy, rather than this sentence tell me.
Something was emerging! A long-body. A green one!
I've never been a fan of exclamation points in a story since it makes it seem as if you're trying to create excitement in a desperate way. It could just be me, but I don't like them.
This first chapter seems too short. I'm afraid I'm going to have to request you work on this one a little bit more before I can allow you to post further chapters.
If you're struggling for length, try adding more description. Right now, the story seems a little choppy. In other words, there's hardly any transition going on and that is going to greatly reduce the length. There isn't much detail going on either. I can guess at what the finer details are, but I'd rather not. Think of things like the body language of the pokemon, how the environment is reacting to the winds, and things like that. I realize you have most of these things, but they feel rushed over. Try expanding on those points, and you should be good.
I feel like the story is there, but not quite the execution. I can imagine encountering a legendary could be a truly terrifying experience, and you do capture some of that fear here, but I feel like there's a lot to improve on. If you could work on this first chapter a little bit more, we'd be good to go.