SlowpokeTail

SlowpokeTail is an item that appears in Pokémon Gold/Silver and Pokémon Crystal. As is name implies, the item is the tail of Slowpoke that has been cut-off or removed. They are considered delicacy that is known for being both nutritous and quite tastey.

As a result they are highly sought after and can be sold for a high price on both the open market (as well as the black market). In the games they can be bought for 9,800 Poke Dollars. It can be sold for extra cash.

In Pokémon G/S/C
The item itself plays somewhat of an important role in the game's plot as Team Rocket takes over Azalea Town and the Slowpoke Well in order to relive the poor Slowpoke's of their valuable tails which they sell on the black market and at the Mahogany Town Poké Mart above their Hideout in G/S/C.

In Pokémon HeartGold and SoulSilver
The SlowpokeTail itself doesn't appear as an item in the remake of the Generation II games, though their are references made to it. Near Route 32's Pokémon Center a man (possibly working for or with Team Rocket) will offer to sell the protagonist (the player) a SlowpokeTail for 1,000,000 (an amount of money that is impossible to aquire in the games), however it can't be bought (as the price too high).

Lagging Tail
Despite not being an item in the game, their is a similiar item called Lagging Tail, a hold item which causes the Pokémon holding it to strike last (an allusion to Slowpoke slowness). A Lagging Tail can be found on Route 47. Also in the wild Slowpoke & Lickitung (possibly a reference to the SlowpokeTails being a delicacy) occasionally appear holding a Lagging Tail.