SlowpokeTail is an item that appears in Pokémon Gold and 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 nutritious and quite tasty.
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 cannot be purchased at first as the price is one more Pokédollar than the player can hold, but later on the price is lowered to 9800, although only in Generation II.
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 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.
The SlowpokeTail itself doesn't appear as an item in the remake of the Generation II games, though there 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 is much too high).
The only way that it can actually be an item is with a cheat device, like Action Replay, though it has no purpose other than as a placeholder.