If the contents of this recent Pokémon leak are to be believed, Pokémon Diamond and Pearl - and every game thereafter - could have been very different indeed.
Dr. Lava, known for sharing and dissecting cut content from a wide variety of Nintendo games, has shared an article exploring a supposed leak that first surfaced just yesterday. The 'leak', which was first dumped on a Pokémon-focused forum on 4chan, showed "hundreds" of sprites that would have been created for already-existing Pokémon in the fourth generation games. These sprites are considerably different to those that made it into the final build, with a large number of Pokémon being treated to noticeable gender differences.
Now, some Pokémon do have different designs depending on their gender anyway, but this leak suggests that Game Freak may have been considering giving almost every single monster a different design, rather than the current handful. In most cases, the female sprites are seen to have smaller body parts like ears, horns, and the like, but there are other differences, too. Some interesting ones include Bulbasaur, where the female has larger spots, and Chimecho, where the male has yellow markings rather than red.
There's even a difference for Charizard, with the female only having one horn:
Of course, it can't be guaranteed that the leak is entirely legit, but as Dr. Lava points out, the evidence certainly leans in favour of it being the real deal. Not only would faking such a large leak take hundreds of hours, but a 2006 issue of Japanese magazine Corocoro - which was sharing new info on the upcoming Diamond and Pearl games - reportedly noted that "Male and female Pokémon will now be visibly distinguishable in most, if not all, cases". The translation, provided by Bulbapedia, goes on to say that "Most female Pokémon will be smaller or will have reduced or lacking physical features.”
Naturally, this never made it into the final game, but it certainly seems like it was the plan originally. Perhaps these designs really do show what could have been?