Why was Heatran chosen to be the first Legendary that could be either Gender?

[KuudereKun]

It's Lore seems to define it as something there would be only one of just as much as the Creation Trio.

It's look and characteristics make it so that a female Heatran does defy Gender norms a bit. But it still feels awfully random.

You'd think they'd save breaking the no Gender for Legenderies rule for like a Duo that embodies Male and Female respectively, and then a third that embodies being Non-Binary. A lot of traditional perceptions of Gender are likely to read that subject into the last three Generation's Mascot Trios, though in the case of Sun and Moon the West and Japan have opposite assumptions about which is which gender wise.

I suppose this sin't the only thing about Heatran that makes it seem like one of the most Ranodm and Superfluous Legendaries. The Sinnoh games provided the Pokemon universe it's Creation myth, and I guess they wanted to have 8 Legendaries for it instead of 7, which is odd since 7 is a perfectly mystical number.

 

[Rocket Zigzagoon]

This was a crazy theory, but I think Heatran was an ancient Pokémon from the stone age or even longer. They were used to be plenty nearby volcanoes, and were considered volcanic counterparts to Aerodactyl, or something. A disaster rendered many pre-historic Pokémon gone extinct, and Heatran were also victims of this disaster. However, since Heatran aren't completely extinct, only a few surviving examples were found around the world.

Legendaries and Mythicals are usually god Pokémon who doesn't want to show biological sexes, due to their famed, feared status. But in rare cases a Legendary Pokémon could give people their true biological genders, to show how they're not whatsoever. Only a few (Like Eon Duo couples/siblings, all-men Forces of Nature) have given out their genders, while others prefer to kept in privacy.

And yes, some Legendary Pokémon were also based on dinosaurs so this is why.

And yet in Pokémon anime, Legendaries are capable of reproduction (contrary to the games, where you will never see them breeding), so in my theory, they hate being forced to breed by humans, and prefer to decide themselves.

 

[Pyrax]

It could be that Heatran is a living fossil, and the specimen we encounter was amongst the last of its kind. It might not be legendary in the same vein as Dialga, Palkia, etc, just an incredibly powerful and endangered Pokémon the player happens to meet.

 

[Poke Dragon]

There are two legendaries that are implied to be a certain gender even though they are still shown as genderless: Solgaleo and Lunala. There are Pokedex entries stating that Solgaleo is male while Lunala is female. Heatran is a little bit of a mystery regarding gender, it can be attributed to the fact that Heatran doesn't come from Pokemon Eggs like most Pokemon and is instead formed by magma deep within the earth.

 

[ZDaemon09]

I always think since it is implied to have more than one Heatran and that they are common in universe compared to other legendary pokemon.
As a result, someone studied on gender differences in their spare time and recorded it in the Dex.