Shinies are rarer than regular pokemon. Some may even fit into your pokemon. Let's take the time to answer questions before you comment.
How Rare?[]
Well, it has a 1/1400 chance to find. It is harder than Auras. So there is a possibly chance that you will find a shiny. With the Shiny Gamepass, you will find it easier. It would be 1/10.
What Shinies are cool?[]
Ahem. Let me talk. The best shinies are mostly black/megas/legendaries. Shiny Mega Charizard X fits perfectly in this section. These shinies are harder to find. Mostly you could find them in patches, or swarms, and roulette.
This is one of the Project: Pokemon wikias pictures, a shiny eevee from Shinypalooza.
Why are shinies rarer?[]
It is because their color changes, like shiny Hawlucha is black.
When were shinies introduced?[]
At Johto, or Generation II. In the Kanto region, it could only be a shiny if it had higher stats. Now, shinies are different colors.
Was there a event for shinies?[]
Yes. It was called "Shinypalooza." It is over now, but wish_z increased the chances for shinies and made mystery codes that were shiny for a whole week.