Since the original 151 in the original Game Boy versions, the Pokemon franchise has continued to expand and evolve through its 25th anniversary in 2021. Not only has that meant expanding the total number of pocket monsters for players to catch, but also the various types of creatures that could be encountered. While most types were introduced inside of the first generation games, others were added periodically, including Dark and Steel types in Generation 2, to Fairy in Generation 6.

In addition to elemental types, Pokemon are typically classified into groups based on how rare they are to find. Legendary and Mythical creatures in Pokemon are available in each generation of game, and typically represent the most powerful creatures that players can come across. With each generation, new creatures are added to the list from the originals like Mew and MewTwo, to the most recent ones seen in Sword and Shield like Zacian, Zamazenta, and Eternatus. Naturally, this power makes them highly desirable and often times difficult to catch towards the end of the game.

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For many Pokemon Trainers, Legendary pocket monsters represent the best of the best. Both incredibly rare and very powerful, these creatures are oftentimes woven into the region's lore or centered around a particular myth that is ultimately proven true. In many games, they're encountered as interactable Pokemon, such as the Legendary Birds from Pokemon Red and Blue, only being able to be captured once per save file. However, starting with Pokemon Platinum, Legendaries would appear once the game's Champion had been beaten such as Ho-oh, Latios, Latias, and more.

From a game development standpoint, there isn't anything specific which qualifies them to be considered a Legendary. Typically, the designation is made through official statements from Nintendo through media or marketing materials.

However, Legendary Pokemon do have unique gameplay traits. Most Legendary Pokemon don't have an assigned gender, simply being listed as unknown. Obviously, there are a few exceptions to this as Latios, Latias, Heatran, Cresselia, Tornadus, Thundurus, Landorus, Kubfu, and Urshifu are clearly specified as male or female. In addition, these Pokemon can't breed or hatch from an egg and the majority can't evolve outside of Type: Null, Cosmog, and Cubfu.

Similar to Legendary Pokemon, Mythical versions are incredibly rare and often times very difficult to get. The main difference between the two is that Mythical Pokemon can't generally be found in the core games through the course of regular gameplay. Unlike Legendaries, which appear at various stages or once the main campaign has been finished, Mythical Pokemon are traditionally announced months and sometimes even years after a specific game has released. In the past, this has included promotions where players needed to not only owned a copy of a specific game, but also by doing something else; such as going to see a specific theatrical Pokemon movie, using a special event item, or utilizing the Mystery Gift feature in newer titles.

The exception to this rule is Mew, the first mythical level Pokemon which was added right before the release of the original Game Boy titles. Game designer and programmer Shigeki Morimoto not only created Mew, but was able to secretly add it into the original Pokemon Red and Green versions two weeks before it launched in Japan. Even Nintendo wasn't initially aware of the newly added creature, though once discovered, its popularity helped make it become a prominent character of the first generation of the Pokemon series and inclusion in movies, TV shows, and manga.

As of right now, there are only 21 Mythical level Pokemon with each generation introducing at least one There is some debate over the creature known as Phione being a Mythical Pokemon due to official sources and statements often contradicting each other. In some cases, it can be considered that the series has 22 Mythical Pokemon. Regardless of Phione's status, the latest Mythical to be added to the series is the Dark/Grass type Zarude, which was officially revealed on Pokemon Day 2020 for Pokemon Sword and Pokemon Shield.

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Source: Bulbapedia