2023-06-16 : Attaching an Equipment with its enters-the-battlefield triggered ability isn't the same as using its equip ability. You don't pay mana for the attachment, and the timing restrictions for equip abilities don't apply.

2023-06-16 : If the target creature becomes an illegal target, the Equipment remains on the battlefield unattached.

2023-06-16 : Mithril Coat doesn't enter the battlefield attached to a creature. Instead, the Equipment enters the battlefield and then a triggered ability attaches it to a creature. You may cast Mithril Coat even if you don't control any creatures.