>>44182415A lot of the circumstances surrounding Silvally are really illogical and stupid. It was made by humans in a lab much like Mewtwo, but while Mewtwo was genetically spliced from the mythical Mew, which could be argued to rub off some of that 'legendary' status onto Mewtwo (which is still silly, people just accept and don't question the reasoning any longer because Mewtwo is a gen 1 Pokemon), we don't know what constituents were used to sew together the chimera that is Silvally, just that its abilities were designed to emulate the legendary Arceus' multi-type adaptability. In terms of game mechanics, Type:Null counts as Silvally's pre-evolution, but narratively Silvally is the exact same creature, Type:Null just hasn't fully realised the potential of its powers due to a lack of training, and the scientists who created the thing just changed its codename upon first deeming the project a failure. The only thing that changes upon 'evolution' is Silvally shedding a manmade restraining helmet and trusting its trainer to take command of its powers.
There should be absolutely no legends in existence in the Pokemon universe describing Silvally, it is a documented creation of the Pokemon world's modern era. Semantically the word 'legendary' has absolutely no business being used to describe Silvally, but Game Freak/TPC seem to be fine with using it to simply denote special 'unique' monster species.