Shiny creatures would be between 1/50 to 1/100 to spawn, so pretty rare to find. They will be a color swap of their original selves and they will have (Shiny) in their name tag which would be golden.
These shiny creatures will apply to all, they will give 2x more drops. But some will deal more damage like cave troll and what not and will be harder to kill and even faster when they walk. And the shiny ones would require 5x more to tame, so pretty hard to tame. And 2x more to feed.
Shininess will apply to all creatures except blockheads.
You can also convert any existing creatures to shiny if you have 50-100 of them, just go to the portal and voila.
And as an extra, if a rainbow unicorn is shiny it will turn gold and run extremely fast and fly higher then the clouds in just 3 seconds if you even manage to tame it.
And also, cave trolls as shiny would require platinum coins.