I imagine this creature story taking places thousands of year's ago in a ancient 'Greek' like region, Latigasus and Shadow Latigasus (Before turning evil) were once friends before they got split up (They both fell to earth thanks to a certain green dragon Pokemon knocking them out of the sky) while Latigasus was found by a good person and raised back to full health, that was not the case for the soon to be Shadow Latigasus, they were found by cruel humans, that caged and abused it, to the point that they broke it's horn. This would trigger the Pokemon into a rage that transformed it into Shadow Latigasus, but due to it already being in a weaken state it was not able to free itself and just angered it's captors further. In its bleakest moment it found her, or more rightly to say she found it, a slave girl had angered her master and had also been locked away as punishment, in a nearby cell to Shadow Latigasus it is their in the darkness of their prison that she made a deal to free it, as long as it agreed to help her destroyed all of mankind, it agree.

Those two would then go raise hell causing mass destruction across the land and go on a quest to find the 'Pot of Creation' and use its powers to change the world, the only one able to oppose them is Latigasus and their friend/hero along with the Pokemon that have joined them.