Haunted Plate Mail

Equipped creature gets +4/+4.
{0}: Until end of turn, Haunted Plate Mail becomes a 4/4 Spirit artifact creature that's no longer an Equipment. Activate only if you control no creatures.
Equip {4} ({4}: Attach to target creature you control. Equip only as a sorcery.)

The List

Magic 2015

Equipped creature gets +4/+4.
{0}: Until end of turn, Haunted Plate Mail becomes a 4/4 Spirit artifact creature that's no longer an Equipment. Activate this ability only if you control no creatures.
Equip {4} ({4}: Attach to target creature you control. Equip only as a sorcery.)

Magic 2014

Equipped creature gets +4/+4.{0}: Until end of turn, Haunted Plate Mail becomes a 4/4 Spirit artifact creature that's no longer an Equipment. Activate this ability only if you control no creatures.
Equip {4} ({4}: Attach to target creature you control. Equip only as a sorcery.)

Although you must control no creatures to activate the “animation” ability, having a creature somehow come under your control in response to that ability won’t stop it from resolving.

It’s possible to activate the first activated ability of Haunted Plate Mail in response to activating the first ability of another Haunted Plate Mail and have them both become creatures until end of turn.

You can activate the equip ability of Haunted Plate Mail while it isn’t an Equipment, but the ability will have no effect. Haunted Plate Mail won’t become attached to any creature.