It seems like a good opportunity make a water/dragon type here yeah. It doesn't really matter at this point that "there is already another pokémon similar to this concept" because they have doubled down on designs and similar concepts several times over by now.
However, Gyarados in Johto kind of did have this kind of function in a way ahah... Although not really mysteriously, other than that it was enraged. Perhaps there could be a mini-plot involving watching for the monster or even luring it out, and that would provide some nice clue to the overall plot or just kind of be part of some cool character arc.
Yeah, I know. I'm having way too high expectations on plot here. It's pokémon, not a proper plot driven game d: sorry.