The deep cuts

ItemHow to getWhy it's niche
Dragon EggSpawns once on the exit portal after the first Ender Dragon killOnly one per world - it's a trophy, not a material. Teleports when clicked; drop a torch under it to collect
Petrified Oak SlabLegacy item - only exists in worlds that predate Alpha slab behaviorStone-textured wood slab; unobtainable in new worlds, a collector's fossil
Poisonous Potato~2% of potato crop dropsMostly a joke item - eating it poisons you. There's a whole "poisonous potato" achievement line
Heart of the SeaAlways inside buried treasure chests (map from shipwrecks/ruins)One use only: crafting a Conduit for underwater breathing and vision
Echo ShardAncient City chest loot in the Deep DarkCrafts the Recovery Compass - points to your last death location
Goat HornDrops when a goat rams a solid blockEight variants with different call sounds; mostly a novelty collectible
Music Disc "5" & "Relic""5" is crafted from fragments in Ancient Cities; "Relic" from trail ruins brushingThe deepest-cut discs - most players never hear them
Enchanted Golden AppleLoot only (dungeons, bastions, ancient cities) - uncraftable since 1.9The "notch apple": Regen IV, absorption and fire resist. Save it for the Warden
Mob HeadsA charged creeper explosion beheads nearby mobsWearable, note-block sounds, and the Wither needs three skulls
Trident~8.5% drop from trident-carrying Drowned (more with Looting)The only non-craftable weapon - Loyalty turns it into Mjolnir
Dragon's BreathBottle the Ender Dragon's breath attack cloudsThe one ingredient standing between you and Lingering Potions
Suspicious StewCraft with a flower, buy from farmer villagers, or milk a mooshroom after feeding it a flowerEach flower hides a different status effect - saturation is the strong play
Name TagUncraftable - fishing, dungeon loot or librarian tradesRenaming a mob "Dinnerbone" flips it upside-down. "jeb_" sheep is the classic
Horse Armor / SaddleBoth uncraftable - loot and trades onlyFamously recipes-free since forever; still the only way to kit a horse
BellSteal from villages or trade with toolsmithsRings for raids and reveals hidden mobs briefly - villages panic when you ring it

FAQ

What's the single rarest item?

Obtainable in normal survival: the Dragon Egg - literally one per world. Among renewable items, mob heads and Music Disc 5 take the most setup.

Are any of these actually useful?

Yes - Heart of the Sea (Conduits), Echo Shards (Recovery Compass), Dragon's Breath (lingering potions) and the Enchanted Golden Apple are all top-tier functional items.

Did you cover the Krillion Minecraft prompt?

Funny you ask - "name any item in Minecraft" showed up in Krillion's daily dive. Petrified Oak Slab was the One-in-a-Krillion answer. See Krillion for how the game works.