Literally just based off the
emoji..
How about a rainbow slime aberration, can appear rarely on any slime.
Has a high amount of HP, so you can actually see what it does instead of just erasing it from existence instantly.
Randomly activates a random slime death effect or ability every few seconds, when killed sets off 3 random slime on death abilities. (Yes, both include even the gold variation, but you won't get the shiny slimy achievement or any other gold related achievements from it.)
When killed, you gain an achievement and item.
"Shinier Slimier."
Kill a rainbow slime.
Item attached to achievement:
Opal whetstone.
Applies a random status effect to an enemy when hitting them with your melee weapon.