"Legendary" really has no solid meaning anyway, since the category of "legendary" includes all sorts of different things...
1: Some of the most powerful Pokémon in existence, which certain groups of ancient humans viewed as gods due to the fact that they seem to be one-of-a-kind (or at least ridiculously rare in the case of Regigigas and maybe the Hoenn trio; like maybe a population only in the double or triple-digits worldwide) and have the power to mess around with a certain element/force of nature. (Dialga, Palkia, Giratina, Arceus, Kyogre, Groudon, Rayquaza, and Regigigas.)
2: Pokémon that are on a similar level as the "god wannabes" but without quite as much mythical baggage (Ho-Oh and Lugia.)
3: Pokémon that are on a similar level as the "god wannabes" that were much more recent in origin, either by a freak mutation or genetically-engineered (Mewtwo and Deoxys.)
4: Pokémon that are much weaker than the "god wannabes," but were still viewed as gods in certain mythologies anyway due to possibly being one-of-a-kind and having a lot of power over certain mental things like emotions and memory (Uxie, Mesprit, and Azelf.)
5: Pokémon that are on a lower level than the "god wannabes" or their equivalents, and in most cases don't come with a lot of mythical baggage either... but they're just so rare and rarely-seen that they were originally thought to not really exist by many people. Basically the "Bigfoot/Loch Ness Monster" equivalents of the Pokémon world. (Articuno, Zapdos, Moltres, Mew, Raikou, Entei, Suicune, Celebi, Regirock, Regice, Registeel, Latios, Latias, Jirachi, Manaphy, Phione, Heatran, Cresselia, Darkrai, and Shaymin.)
So whether Heatran is a "legendary" Pokémon or not really depends on how you define "legendary". If you're going to throw out Heatran, you'd have to "de-legendary-ize" at least a few others, too... if Heatran doesn't count as "legendary" just because it's non-unique and has a defined gender, then why would Latias/Latios (who have those exact same features!) still count as "legendary?"
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Also... the Japanese version actually has two or three different categories of legendary Pokémon, rather than calling everything "Legendary." This might be part of the reason why there's so much disagreement over what's "legendary" and what isn't.
If I remember right, there's "Phantom" Pokémon (basically group #5 above, the kind that's just so rare that a lot of people don't believe that they really exist), "Legendary" Pokémon (Ho-Oh, Lugia, Kyogre, Groudon, and so on; basically group #2 with parts of #1 mixed in), and then a third group that's exclusively for Sinnoh's mythical Pokémon (Dialga, Palkia, Giratina, Arceus, and maybe the lake trio... I can't remember for sure.)
Somebody that's more familiar with the Japanese version can probably fill in which one goes in which group better than me (I honestly can't remember if the "Sinnoh mythical Pokémon" group includes all the 4th-gen legendaries, or just the ones related to the whole Arceus/Dialga/Palkia story, and I have no idea where Mewtwo, Deoxys, and various others fit in)... though I think I at least got the basics of the three groups down.