Unlike usual Wild Pokemon Encounters found in the game that only sees players having to throw Poke Balls in order to make a capture, Legendary Pokemon Encounters work a little differently. They remain similar to these other encounters, but they are more akin to the battle type encounters found in previous Mainstream Pokemon Games. You see, before being able to capture a Legendary Pokemon like you would any other Pokemon found in the game, you must first defeat it in battle.
The battle against these Legendary Pokemon will be just like Wild Pokemon Encounters of old; however, a player will only have five minutes to defeat a Legendary Pokemon in battle. If you don't succeed at defeating a Legendary Pokemon within the five minute time limit, you will have to battle it all over again under the same perimeters. After defeating a Legendary Pokemon in the battle portion side of the encounter, the capture screen will be triggered and you can proceed with trying to catch it like any other Wild Pokemon.
There are a grand total of seven Legendary Pokemon a player can obtain: Articuno, Zapdos, Moltres, Mewtwo, Mew, Meltan and Melmetal. However, Legendary Pokemon Encounters do not apply to Mew, Meltan and Melmetal since they are Mythical Pokemon and cannot be found in the wild. Furthermore, Mewtwo, being the strongest Wild Pokemon in the entire game, will have a special feature that will make it more difficult to capture after it has been defeated in battle.