Legendary and Mythical Pokemon being used by a trainer isn't as rare as they used to be in the anime. Hence, it's actually not a rare sight anymore. There are even moments when unnamed trainers are seen with a Legendary. A good example is the random trainer who had Heatran in the Lily of the Valley Conference.
This article will highlight some of the more notable appearances from Legendaries and Mythicals as they appear in the anime (particularly under a trainer's command). There is a good amount of content to talk about here, so buckle up.
Note: It won't include Ultra Beasts, as they're technically a separate group from Legendary and Mythical Pokemon.
10 times a trainer used Legendary or Mythical Pokemon in the anime
10) Articuno (by Noland)
The Battle Frontier arc featured a plethora of Legendary Pokemon, one of which was Noland's Articuno. He didn't technically catch Articuno, but this Pokemon agreed to fight on his behalf against Ash's Charizard. Articuno did exceptionally well despite the type disadvantage, but it ultimately lost to Ash's Charizard in the end.
Thus, Ash earned his Knowledge Symbol and proceeded with his journey through the Battle Frontier.
9) Regirock (by Brandon)
The first Legendary Pokemon ever caught and used by a trainer in the anime was Brandon's Legendary Titans. All three of them are exceptionally strong, although it's worth covering each of them separately. Brandon's Regirock has defeated Ash's Sceptile and Paul's Hariyama, Electabuzz, Ursaring, and Magmar. It was also seen in a flashback defeating the Drapion owned by Paul's brother.
Normally, Regirock is seen with the other two Legendary Titans that Brandon also owns.
8) Registeel (by Brandon)
Brandon's Registeel would end up defeating Ash's Torkoal in the anime episode, Overjoyed!, as well as Paul's Nidoking in A Pyramiding Rage! His Registeel was seen as more defensive than his Regirock and didn't get as many dominant showcases by comparison.
Still, it was used regularly alongside Regirock and Regice when Brandon needed the trio's power.
7) Regice (by Brandon)
The final Legendary Pokemon that Brandon owned was Regice. It would be caught offscreen by him during the Battle Frontier arc, yet it was almost as powerful as the previous two Pokemons. However, it was also the only one of Brandon's Legendaries that was seen losing, as it lost to Pikachu in Pace - The Final Frontier!
This Regice would appear in future anime episodes where it would defeat Paul's Lairon, along with teaming up with the other Regis to fight Regigigas.
6) Latios (by Tobias)
Ash losing to Tobias in the Sinnoh League was an unexpected way to stop an otherwise impressive performance by the former trainer. Nonetheless, Tobias infamously had Latios and Darkrai (a Legendary and a Mythical Pokemon). He wouldn't usually use Latios, but he had to when Ash defeated his Darkrai.
This Latios easily defeated Ash's Sceptile and Swellow before drawing with Pikachu. Fans never found out who Tobias's four remaining mons were, but they were likely just as rare as Darkrai and Latios.
5) Darkrai (by Tobias)
Darkrai's role in the anime was briefly alluded to in the previous entry. It was Tobias's ace, having easily defeated every Gym Leader and other trainers in the Sinnoh League on its own. How he caught this Mythical Pokemon is unknown, which isn't surprising, given that Tobias barely had any screentime.
Unsurprisingly, Tobias won the whole Lily of the Valley Conference. His most challenging bout was against Ash, where his Darkrai defeated his Heracross, Torkoal, and Gible. It finally lost to Ash's Sceptile, but it would beat a random Magmortar in the finals to help Tobias win the whole conference.
4) Mewtwo (by Giovanni)
Perhaps one of the most famous examples of a trainer using a Legendary Pokemon in the anime is when Giovanni used Mewtwo to destroy Gary Oak. This moment happened in the episode, The Battle of the Badge. Gary Oak easily defeated Giovanni's first two mons and was over his head when he thought victory was assured.
Sadly for him, Giovanni had an ace up his sleeve. Queue Mewtwo showing up with some strange armor. The battle ends with Gary's Nidoking and Arcanine easily being beaten by Mewtwo, thus serving as a brief teaser of Mewtwo's power in the upcoming movie.
3) Suicune (by Goh)
Goh has caught an absurd amount of Pokemon for an anime character. One of those mons was the Legendary Suicune. Goh caught it in Johto, and although he wanted to release it, Suicune decided to continue being under his control. Suicune would still roam around as if it were a wild Pokemon, although it would return in a later episode to help purify some water near Cerise Park.
When it's not under Goh's control, this Suicune usually purifies some water somewhere. Funnily enough, Goh only caught this Legendary Pokemon to save it from some hunters who wanted to catch it.
2) Silvally (by Gladion)
It's easy to forget that Silvally and its pre-evolution, Type: Null, are Legendary Pokemon. They're nowhere near as powerful as other behemoths like Kyorge, Mewtwo, or Arceus, but they're still labeled as Legendaries by the Pokemon Company.
Silvally was one of Gladion's five mons shown to be used by him, and it had the impressive ability to sense Ultra Beasts and their Ultra Wormholes. It was arguably his strongest mon, as it was capable of defeating Ash's Melmetal in the Manalo Conference.
1) Melmetal (by Ash)
There used to be a type when the main characters wouldn't own such rare Pokemon like Melmetal. However, that has changed a lot in the recent anime seasons, leading Ash to catch a Meltan that eventually evolved into a Melmetal. This was the first time that any main character owned a Mythical Pokemon.
Due to Melmetal accompanying Ash, it received far more screentime than most of the previous entries on this list. His Melmetal has usually been seen as being very strong in its appearances in the anime, although Ash still decided to leave it at Professor Kukui's home alongside his other Alolan mons.