I have seen
-2 slimes(no superflat)
-A spider jockey on the first night of a wordl
-pink sheep.
Now my friend and his family, they are EPIC!
all of this are legit. No mods, no pre-releases, all of them naturally spawned!
-green sheep (there is NO coding for that somehow it spawned)
-a village house fusioned with a dungeon. the dungeon had an ENDERMAN spawner
-a block of obsidian, and only 1 block, in the middle of a lava lake (no water)
Now what would be really epic (tell me if you see this)
-a UFO (bukit plugin) hitting a skeleton that shoots an arrow to a pig wich fals down on a pressure plate activating a mob arena wich sends a charged creeper that follows a player until the player falls on a chicken pit. the creeper blows up, killing the chickens and setting a secret tnt wall off wich blew up a castle.
THAT IS POSSIBLE