Historical Rules · 15 July 2011
Rule 301.5c
Official source text from the 15 July 2011 Rules release
An Equipment that's also a creature can't equip a creature. An Equipment that loses the subtype "Equipment" can't equip a creature. An Equipment can't equip itself. An Equipment that equips an illegal or nonexistent permanent becomes unattached from that permanent but remains on the battlefield. (This is a state-based action. See rule 704.)