There is a lot of species in the Pokemon World, some truly might be rarer than others. But legends are told of Pokemon who rule or represent certain things. Pokemon that meets these requirements are called Legendary Pokemon. They are incredibly rare and often very powerful Pokemon. Some will show themselves to worthy trainers. Like the legendary Ho-oh of the Johto region who will only come out when a trainer has proved that they trust their Pokemon and together as a team are unstoppable. You also must have the rainbow wing and you may challenge them.
Some Legendary Pokemon lurk around the region and you may run into them in the wild at random notice. For example, Entei, Suicune, and Raikou can be done in the wild but they will flee at the first chance they get, it is tough to capture these types of legendary Pokemon.
Others may be called forth using tools and equipment. Team Galactic used the Red Chain, Lustrous Orb, and Adamant Orb to call forth Dialga and Palkia. They are ancient and trainers who see such Pokemon are usually very lucky and determined. To capture such a Pokemon, it takes a lot of work. Legend Pokemon are usually at very high level for power and will not go down without a long fight. If a trainer has the powerful Master Ball, which is a Pokeball that can capture any Pokemon without failure, the legendary Pokemon can be made without a fight. However, you can not just go to the PokeMart and buy fifty master balls. Master Balls are extremely rare, most trainers only succeed in getting one single Master Ball on their journey as a reward for working so hard for their goal. But even with that said, Master Balls, just like Legendary Pokemon, are rare and usually lucky for a trainer to receive. You must use the Master Ball wisely, there is for sure more than just one Legendary Pokemon in the world, only one you can use the Master Ball on while the rest you'll have to capture in battle! It may be a tough task, but if you want to catch them all, its a task you'll have to face on the Pokemon Journey.