TR31 is an item in the “All machines” category (TMs and HMs). It costs ₽10000.
In-game descriptions
| Sword / Shield | The target is slammed with a steel-hard tail. This may also lower the target’s Defense stat. |
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Move taught
| Sword / Shield | Iron Tail |
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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