So, what is the Dragon-type?
Dragons are the archetypal RPG enemy, despite appearing in so few actual fantasy stories. Classically, Dragons were gods of religions, and later messengers of gods, until the emergence of monotheism which rendered them to enemies of God, and later God's followers. Reemergence of fascination with dragons seem to be more facilitated by expansion of discoveries involving dinosaurs, which are generally considered to be ideal ancestors of dragons. Another reason is that while other stories required dragons to be relevant or well designed, RPGs, tabletop and video, did not require that, they just required an artwork and some stats.
Dragons' historical connections with elements of nature and their modern and Japanese association with serpents, long fishes and amphibians became the core basis of the Dragon-type in Pokémon, but of course this was one of the types SMT did before Pokémon. As time passed on however, Dragon just became a descriptive type for powerful serpents that did not have other types, as well as other mythical beasts that did not fit into other paranormal types. This made Dragon-type more esoteric, but also much more weaker, with the final nail in the coffin coming with the introduction of Fairy-type.
The History of the Dragon-type:
In the original game, there was one Dragon-type, Dragonite, which was also the pseudo-legendary of the game, and appropriately rare and hard to raise. And there was only one Dragon-type move, Dragon Rage, which was a fixed damage move that was a complete joke in later levels, but completely destructive in the early game. Surely, this would mean there wouldn't be anyone training this, right? Well, no. Lance was a fully fledged Dragon-type master who was the original champion before Blue beat him, and his team was focused on species who learned the Dragon Rage TM, because of course it was TM.
This absurdity was caused because Dragon-type was meant to be the ultimate type, the type that was so rare and elusive that no other species would reach it. But of course, this is Pokémon. You are supposed to catch 'em all and train them to be the very best. So, "absolute final boss the type" could not work feasibly. But just letting it out would unbalance the game, which is why you have several species that have characteristics of a Dragon-type, without being a Dragon-type.
The next generation introduced another Dragon-type master, the cousin of Lance, Clair, or as I like to call her "Water Master Clair". And of course, you had another Dragon-type, which was the cross-generational evolution Seadra, Kingdra. Kingdra was the sole reason Seadra was not granted a decent Special Defense, of course. And of course this Kingdra did not get the strongest Dragon-type move Outrage, despite being introduced in this generation. As usual.
The new Dragon-type moves introduced here were Twister, which was largely for Kingdra, Dragon Breath, which was meant to replace Dragon Rage, and Outrage, which served as the strongest move of the type, despite being a Thrash clone. The new Steel-type also served to nerf Dragon-types, but Dragon-types did not have much trouble with it, because they were not good enough.
The new generation, the third one, had a major Dragon-type legendary, so they needed to fix stuff. The first enlargement of the type created a bunch of weird species, which would not be repeated. The first one was Altaria which became archetypal for early Dragon-types. Another one was Flygon, which was the ultimate mid-game Dragon-type. To compensate this, Bagon was not available until obtaining Waterfall, which was impossible without beating the final Gym. Rayquaza was initially unavailable in the early game, but became available in Emerald, which became the archetypal distribution of third games.
There was only two new moves, Dragon Dance, which was one of the strongest buffing moves in the game. Sadly, Dragon-types were special, so you wouldn't abuse your STABs here. The other Dragon-type move was Dragon Claw, which served as the upgrade to Dragon Breath. There wasn't much from Abilities, sadly. Just tons of Levitate.
Fourth generation did not stop the esoteric nature of the Dragon-type. The new mascot legendaries looked like anything but dragons, while being clearly dragons. The sole common Dragon-type was just the pseudo-legendary, just like in the original games, but you could obtain it by the second gym and evolve it around sixth. Of course, this was also one of the strongest Dragon-type, Garchomp, so have fun breaking the game.
Due to physical-special split, Dragon Claw no longer worked as a replacement to Dragon Breath. So, you needed a proper Special move, which was the Dragon Pulse. Outrage was buffed to the level of Double-Edge, as Dragon Pulse covered the 90 BP role. Outrage also became a Tutor, alongside the one-off Twister. The fan made Draco Meteor also appeared as an Overheat clone, but it was only accessible to Dragon-types. Another new move was introduced, Dragon Rush, but it served as little more than a pseudo-signature move for Gible-line.
The new Dragon-types introduced in this generation are special, because they actually look like Dragons, despite their esoteric origins. One of them was Axew, which was basically a budget Gible, Druddigon who was a mid-game Dragon-type with Altaria-like stats, and Hydreigon, which was only accessible in the absolute final point of the game. The legendaries were the first in the franchise that actually wanted you to catch them, but that was because you went toe to toe against their opposite. Because of this change, Dragon-type was made into its own type in the middle of the generation in TCG. Its non-elemental status remained because TPCi is too cheap to make Dragon-type energy cards.
Gen V introduced only two moves, Dual Chop, which became available for Fighting-types. The other move was Dragon Tail, which was basically Roar combined with a Dragon-type mid-game move. This generation introduced a Dragon-esque Ability, Multiscale, but it was only available to two species, one of which wasn't even a Dragon-type.
Sixth gen was the big Dragon boom, due to Fairies. With fairies, Gen 6 could go with the early Dragon-types introduced in other generations without repercussions. This could only mean good things... Anyways, because Dragon-type was no longer the strongest type, and just a type now, many species that could have gained Dragon-type previously gained it in their Megas. Dragon also came back for the third legendary, though this legendary was an inversion of Rayquaza, and a pseudo-legendary, which was more defensive than others. The late game Dragon-type was not a primary Dragon-type, and evolved from a Zubat clone, and another Dragon was one of the fossils. Of course, you had the dragons from previous generations in early routes as well. Brilliant.
The sixth generation did not introduce any new moves for Dragon-type, but Dragon Pulse was nerfed.
Seventh gen basically said fuck you to the type, and made two Dragon-types that were version counterparts to each other, one being late-game, other being mid-game, another pseudo who was the weakest of the type, new forms for Zygarde because Z got cancelled, and Guzzlord, which is literally a stomach. There is also Alolan Exeggutor, which exists so Gen I has another Dragon-type.
All moves introduced in this generation were signature moves, with one of them getting a Z-move for USUM. I guess Fairies killed the type.
The Overview of Dragon-types:
Dragon-type is the prime example of why conventional boss elements do not work in Pokémon. In Pokémon, anything you can see, fight and encounter is catchable. Even the items. With this in mind, there is absolutely no reason for a type to be the be all end all of all types. Because certain regions will introduce differing amount of types for sake of balancing and fitting the region. With this in mind, no type can be truly the best, like no one ever was. But no type can be truly the worst. And all types have to be like this in all areas, moves, species, stats and abilities. Everything has to be absolutely and aggressively mediocre. No asymmetric balancing here.
And when this is not met, you have issues. And Dragon-type has issues. In its original form, Dragon-type was a type with decent resistances and large movepool, and great STAB. But the drawback was that it had hindering dual types and no coverage. But with Fairies introduced, Dragon-types are no longer a proper end boss type, but rather a generic wallbreaker type. Basically, they are an outgrowth of Fire-type these days.
However, Dragon-type still retains the great stats and thus has great imbalance within itself, with the absolute strongest Dragons remaining the strongest species, and the older species being weak enough to be absolute lowest tiers, and this goes to mascot legendaries themselves. While this can be easily fixed probably by giving Dragon-type a proper strength against, without that Dragon-type will never reach its full potential as a generic type and will forever remain a gimped type with big stats and novelty coverage for other species from other types.