Lagging Tail is an item in the “Bad held items” category (Items). It costs ₽4000.
In-game descriptions
| Diamond / Pearl · Platinum · HeartGold / SoulSilver · Black / White · Black 2 / White 2 · X / Y · Sun / Moon · Ultra Sun / Ultra Moon · Let’s Go, Pikachu! / Let’s Go, Eevee! · Sword / Shield | An item to be held by a Pokémon. It is tremendously heavy and makes the holder move slower than usual. |
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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