Although Shiny Pokémon are usually pretty rare, there are a few instances throughout the Pokémon main series games where you are guaranteed to encounter or receive a specific Shiny Pokémon.
Shiny Gyarados (GSC/HGSS)
In Gold, Silver & Crystal, as well as HeartGold & SoulSilver, you are actually required to fight against a Shiny Gyarados in the Lake of Rage north of Mahogany Town.
This Shiny Gyarados is always level 30 and makes a great addition to your team when you get it. Even if you don’t use it for battle (let’s say you already have a dedicated Water-type), it can learn a lot of helpful HMs that you might not want to teach your other teammates, like Strength, Whirlpool, Waterfall, and even Surf.
Shiny Gible (Black 2)
In Black 2, after clearing the Black Tower and defeating Benga, he will reward you with a Shiny Gible.
Shiny Dratini (White 2)
In White 2, after clearing the White Treehollow and defeating Benga, he will reward you with a Shiny Dratini.
Shiny Haxorus (Black 2 & White 2)
After seeing all 300 Pokémon in the Unova Pokédex, you can talk to Professor Juniper in Nuvema Town and she’ll reward you with the Permit. You can take the Permit to the airport in Mistralton City to fly to the secret Nature Preserve.
The Nature Preserve has a few rare Pokémon as it is, but the true reward is the stationary level 60 Shiny Haxorus waiting for you in the tall grass. You can save your game before fighting it. Don’t worry, as it doesn’t have any moves that cause recoil damage, although you may have to watch out for Guillotine if your Pokémon are level 60 or lower.