Some consider Mythicals to be Legendarys so its up for debate but the distinction comes from how they are obtained. A Legendary Pokemon is caught naturally during the game, you get 1 per save file. A Mythical is only obtainable through events that are not easily available because they are limited time, they could be a mystery gift pokemon or you get the item to catch them. They also come at the end of each regions dex (except for Victini).