ネズミのしっぽ [nezumi no shippo] or 'rat tail' in Japanese. One of the longest-standing references in Final Fantasies, it being an important key item in Final Fantasy I.
Final Fantasy I
Obtain: beat Citadel of Trials
Use: give to Bahamut in order to unlock the advanced jobs
Final Fantasy IV
Obtain: ?
Use: give to the tail collector in exchange for Adamantite
Final Fantasy XII
Buy: - (sell: 224 gil)
Use: Megalixir
Drop: Dire Rat, Lab-Rat
Poach: Dire Rat
Description: A common alchemical ingredient. Rumored to be quite tasty when properly prepared.
Revenant Wings
Buy: 1,500 (sell: 750 gil)
Shop: Material Shop, chapter 6 onwards
Use: Earthsaber, Elven Bow, Pyroblade, Rune Blade
Obtain: mission 4-2 (harvest 67%), 5-2 (harvest 67%), The Sword in the Shell (harvest 67%), Cebe Summoning Melee (harvest 67%), Gugoza Falls Summoning Melee (harvest 67%)
Description: Tail used prominently in alchemy. Putrid flavor.
Other: low-grade Braided Twine
Final Fantasy XIV

Equip: level 7 (optimal), FSH
Buy: - (sell: 2 gil)
Drop: various gnats
Obtain: buy 20 for 30 seals from your Grand Company
Type: Bait, Stack: 99
Description: The thin tail of a rat, often carried by young Lalafell boys as a good luck charm. An all-purpose bait.
Final Fantasy Tactics

Obtain: randomly from one of the following Errands: Gariland Magick Melee, Mount Gulg Mother Lode, Zaland Embassy Antiques, Abandoned Mine, Clockwork Faire, Salvage Expedition, Zerro's Challenge, Guard Duty, Yardrow Melee, Gleddia Isle, Lettre d'Amour, Count Minimas (1), Count Minimas (2), The Durga, Endless Caverns, The Falcon, Father's Nightmare, Himca Cliffs, More Coal Miners Wanted, Ore of the Gods, Past Glory, Rhana Strait, True Romance, Zerro's Return, Zerro Strikes, Zerro Strikes Again, Appraisal, Gysahl Greens, Historic Revolt, Lionel Emissary, Second Coal Mining Expedition
Type: Artefact
Description: Long used among sorcerers as a magickal reagent, this exquisite delicacy was once the catalyst for a full-scale war. It has an extremely potent flavor.
Tactics Advance
Obtain: missions Words of Love and Lucky Night
Use: in missions Lucky Charm and Armor & Turtle
Grimoire of the Rift

Buy: - (sell: 20 gil)
Use: Four of Spades, Adamant Vest
Reward: A Voice from the Well x2 (once)
Type: Skin, Rank: —
Description: The long tail of a desert rat. Thought to bring good luck, provided one isn't a rat.
Final Fantasy Tactics S

Buy: - (sell: 50 gil)
Use: Bell, Speed Ring
Obtain: Rhode Rum Region/White Highroad East
Description: A somewhat uncommon skin
Crystal Chronicles
Effect: DEF+2
Obtain: Veo Lu Sluice, cycle 2
Type: Artifact
Other: as 'Rat's Tail'
Category: Item