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? Edit
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? Edit
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.
Why are shinies rarer? Edit
It is because their color changes, like shiny Hawlucha is black.
When were shinies introduced? Edit
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? Edit
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.