Slowpoke Tail is an item in the “Unused” category (Key Items).
In-game descriptions
| HeartGold / SoulSilver · Black / White · Black 2 / White 2 | A very tasty tail of something. It sells for a high price. |
|---|---|
| X / Y · Sun / Moon · Ultra Sun / Ultra Moon · Let’s Go, Pikachu! / Let’s Go, Eevee! · Sword / Shield | A very tasty tail of something. It can be sold at a high price to shops. |
Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h