Like @Luplayz said, it's happened before, so it's not really surprising that it happened again. With that said however, I agree with you that I think they should've done something different, typing wise at least. A combination of Dark, Ghost or Fighting (and to an extent Fairy or Psychic) are all types I could see fitting for Zarude (hate the name btw). To me it kinda looks like it was supposed to be in a trio with Oranguru and Passimian, got cut and then they decided on a Tarzan'esque movie, needed a Mythical, went through some old designs and picked that one. I don't think it looks bad, but I just don't think it looks special either.
Rillaboom looks more Mythical than Zarude if we’re talking aesthetics that one would assume a Mythical Pokémon to have. But I like the design. It’s growing on me.
Melloetta is always normal type, Psychic is replaced with fighting in its pirouette forme, therefore, we always have had a mythical pokemon with the normal type in all of its forms since gen 5
Maybe I didn't word it right, if I didn't then I'm sorry. I know Meloetta is always part normal. I mentioned her being part Psychic or Fighting depending on her form. Mentioning those two is just to differentiate them from what would be a pure Normal type 100% of the time, regardless of secondary typings or form changes.
Not to mention Normal/Ghost would be a really interesting typing. Immunity to Normal, Fighting, and Ghost, and a weakness to only Dark. Every new gen I want them to jump on this idea, lol.
