Lightning Greaves
Artifact — Equipment
Equipped creature has haste and shroud. (It can't be the target of spells or abilities.)
Equip 
After lightning struck the cliffs, the ore became iron, the iron became steel, and the steel became greaves. The lightning never left.
イラスト：
Tarkir: Dragonstorm Commander#102アンコモン英語適正
裁定
You can't simply unequip Equipment from a creature. If Lightning Greaves is attached to the only creature you control, you won't be able to attach other equipment to it (or target it with anything else) until you have another creature onto which you can move Lightning Greaves.
If a creature enters the battlefield under your control and gains haste, but then loses it before attacking, it won't be able to attack that turn. This means that you can't use one Lightning Greaves to allow two new creatures to attack in the same turn.