All Pokemon types are not created equal, and that shows in the Legendary Pokemon.
Some types just have more than others, and there's a big difference between the Legendary-heavy types and the ones that need more love in that department. It can also be interesting to look at how the types affect the Legendary Pokemon they produce, and how they tie into the overall lore of the Pokemon series. This list will look at all 18 types and how many Legendaries they have.
Note: For the purposes of this list, Mythical Pokemon and Ultra Beasts will be considered, as will alternate enhanced formes (Crowned Zacian and Crowned Zamazenta) and regional variants (Galarian birds). However, Arceus and Silvally will only represent the Normal-type.
18 Bug (3)
Mythicals: Genesect
UBs: Buzzwole, Pheromosa
Aside from Mythicals and Ultra Beasts, the Bug-type is still barren of Legendary Pokemon. That doesn't help a type that's considered to be one of the weakest in the franchise. Even though Genesect, Buzzwole, Pheromosa are plenty powerful in their own right, them being the strongest Bug-types ever is pretty rough when compared to other types.
17 Poison (3)
Legendaries: Eternatus
UBs: Nihilego, Poipole, Naganadel
In a stark contrast, the Poison-type's claim to fame is the strongest Pokemon to ever grace a screen: the absolutely bonkers 1125 base stat total Eternamax Eternatus. You can't actually use it in battle, but the Poison-type will take what it can get in terms of Legendary Pokemon, especially since Nihilego, Poipole, and Naganadel's status as Legendaries is debatable.
16 Ground (3)
Legendaries: Groudon, Landorus, Zygarde
There are a dismal three Ground-type Legendary Pokemon, which can be a bit surprising depending on who you ask. Regardless, said three are incredible in terms of power and lore.
Groudon is the literal god of land, Zygarde has a lot of mythos surrounding it from both the Sun and Moon games and the anime, and Landorus is a competitive battling staple.
15 Rock (5)
Legendaries: Regirock, Terrakion
Mythicals: Diancie
UBs: Nihilego, Stakataka
Most people consider the Rock-type to be pretty subpar compared to the others, and that's certainly true with the Legendaries it has. There are only two to name, and five if you include Mythicals and Ultra Beasts. They're all pretty decent as far as Legendary Pokemon go, but if you're looking for a Legendary-abundant type, look elsewhere.
14 Normal (5)
Legendaries: Regigigas, Type: Null, Silvally
Mythicals: Arceus, Meloetta
The Normal-type has a bit of a rag-tag group of Legendaries and Mythicals, which is interesting to see. No other type can claim it has a pop star, a gigantic mecha with a taco on its head, an artificial Pokemon line, and the literal god of the universe on its roster.
13 Ice (5)
Legendaries: Articuno, Regice, Kyurem, Glastrier, Ice Rider Calyrex
The Ice-type comes in with a solid fivesome of Legendaries. While Ice Rider Calyrex is merely an alternate forme, Kyurem comes with three formes of its own, two of which are banned and the remaining base forme being a competitive staple. The other three, while not necessarily viable, stand out in their own way design-wise.
12 Grass (7)
Legendaries: Virizion, Tapu Bulu, Standard Calyrex
Mythicals: Celebi, Shaymin, Zarude
UBs: Kartana
The Grass-type Legendaries can be a bit underwhelming in terms of competitive usage. Celebi has fallen out of viability, and the newer Zarude and Calyrex (in its standard form) have too many weaknesses to be used regularly. The one exception is the lone Grass-type Ultra Beast, Kartana, who puts in work with its massive 181 attack stat.
11 Dark (7)
Legendaries: Galarian Moltres, Yveltal, Single Strike Style Urshifu
Mythicals: Darkrai, Hoopa Unbound, Zarude
UBs: Guzzlord
The Dark-type got its time in the spotlight kinda late, with Darkrai being the oldest Pokemon in this entry. In fact, only Darkrai, Yveltal, Guzzlord, and Zarude are full-time Dark-types, with Moltres being a Fire-type in its Kantonian variant, Hoopa having a Ghost-type Confined forme, and Urshifu having a Water-type Rapid Strike Style forme.
10 Water (8)
Legendaries: Kyogre, Palkia, Tapu Fini, Rapid Strike Style Urshifu
Mythicals: Phione, Manaphy, Keldeo, Volcanion
The most populated type in the game has a solid group of Legendary Pokemon to complement its abundance. While that group is obviously led by the god of the seas in Kyogre, there are a select few that stand out by being Water-type without necessarily having an obvious connection to the ocean, such as Palkia, Keldeo, and Urshifu.
9 Ghost (8)
Legendaries: Giratina, Lunala, Dawn Wings Necrozma, Spectrier, Shadow Rider Calyrex
Mythicals: Hoopa Confined, Marshadow
UBs: Blacephalon
It's not surprising to see a type based on the afterlife have a lot of Legendary Pokemon on its roster, and strong ones at that. Six out of these eight Pokemon are banned from regular competitive battling, and while Hoopa hasn't been added to Sword & Shield, Blacephalon has seen quite a bit of use since its introduction.
8 Electric (8)
Legendaries: Zapdos, Raikou, Thundurus, Zekrom, Tapu Koko, Regieleki
Mythicals: Zeraora
UBs: Xurkitree
The Electric-type Legendaries are just about what you expect: a lot of (mostly) fast offensive Pokemon. The pool literally includes the fastest Pokemon in the game (Regieleki) and a few other Pokemon that are known competitively for their speed and attacking power (Tapu Koko, Xurkitree, and Zeraora).
7 Fairy (8)
Legendaries: Xerneas, Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini, Zacian
Mythicals: Diancie, Magearna
Rounding out the four-way tie of eight Legendary Pokemon each is the newest type in the game. Game Freak made sure to fill out the pool of Fairy-type Pokemon as quickly as possible, and the Legendaries are no exception. Each generation that the Fairy-type has existed also introduced a Legendary to represent it.
6 Fire (9)
Legendaries: Moltres, Entei, Ho-Oh, Primal Groudon, Heatran, Reshiram
Mythicals: Victini, Volcanion
UBs: Blacephalon
The Fire-type has been one of the most popular, and the nine Legendary Pokemon certainly help that cause. The older ones such as Moltres, Entei and Ho-Oh have become iconic to Pokemon fans, and newer ones like Reshiram, Volcanion, and Blacephalon put a nice conceptual spin on the type that can't be seen anywhere else.
5 Fighting (12)
Legendaries: Galarian Zapdos, Mega Mewtwo X, Cobalion, Terrakion, Virizion, Zamazenta, Urshifu
Mythicals: Keldeo, Pirouette Meloetta, Marshadow
UBs: Buzzwole, Pheromosa
There's a very nice collection of Legendary Pokemon from the Fighting-type, encompassing all shapes and sizes. The Swords of Justice are an underrated quartet, while Marshadow is one of the most popular Mythical Pokemon ever. To top that group off, Generation VIII gave it two mascots: Zamazenta for Pokemon Shield and Urshifu for the Isle of Armor DLC.
4 Flying (12)
Legendaries: Articuno, Zapdos, Moltres, Lugia, Ho-Oh, Rayquaza, Tornadus, Thundurus, Landorus, Yveltal
Mythicals: Shaymin Sky Forme
UBs: Celesteela
For a type as supplementary as the Flying-type, quite a few Legendary Pokemon will be given the type just for having wings or the ability to fly. In fact, most Legendaries are grouped together from a common association with the sky, such as the Legendary birds and the Forces of Nature.
3 Steel (15)
Legendaries: Registeel, Dialga, Heatran, Cobalion, Solgaleo, Dusk Mane Necrozma, Crowned Zacian, Crowned Zamazenta
Mythicals: Jirachi, Genesect, Magearna, Meltan, Melmetal
UBs: Celesteela, Kartana
Even though the Steel-type is more commonly associated with defense, the Legendary Pokemon tell a different story. There's a surprising amount of diversity from the Pokemon listed here, from cute Mythicals like Jirachi to god-like behemoths like Dialga.
2 Dragon (15)
Legendaries: Latias, Latios, Rayquaza, Dialga, Palkia, Giratina, Reshiram, Zekrom, Kyurem, Zygarde, Ultra Necrozma, Eternatus, Regidrago
UBs: Guzzlord, Naganadel
Dragons are, by nature, mythical creatures, so the Pokemon-type that's based on it will get a lot of extremely powerful Legendary Pokemon. In fact, Dragon-type would most likely be the best when it comes to strength, since the Pokemon listed here include quite a few 680+ base stat total box art mascots.
1 Psychic (22)
Legendaries: Galarian Articuno, Mewtwo, Lugia, Latias, Latios, Uxie, Mesprit, Azelf, Cresselia, Tapu Lele, Cosmog, Cosmoem, Solgaleo, Lunala, Necrozma, Calyrex
Mythicals: Mew, Celebi, Jirachi, Deoxys, Victini, Hoopa
It's kind of odd that the Psychic-type is this saturated with Legendary Pokemon, to the tune of 22 different species. It's probably thanks to how vague the Psychic-type can sometimes be. It's more explicit with Pokemon like Mewtwo and Calyrex who have immense brainpower, but otherwise, it's a bit of a mixed bag. Why is Deoxys even a Psychic-type?