Shimmer Slime

None of these exist anywhere in the world on their own. You get one by putting an ordinary slime into Shimmer or spraying it with the Shimmer Gun, which usually happens by accident while you are working around a pool. A long list of slimes refuses the change: the boss ones, lava and dungeon variants, spiked types, gastropods, crystal and bouncy slimes, and anything a statue produced.

The converted slime picks up the liquid's habit of throwing projectiles back. Fire a bullet or a magic bolt into one and there is a good chance it returns along the line you sent it, which is a real hazard when you have made several without noticing. Melee swings and minions pass through the problem entirely. Royal Gel still pacifies it like any other slime, and it is not on the checklist for the Gelatin World Tour, so there is no completion reason to keep one alive.