Umbral Mantle

• WOTC Official Reminder Textofficial rules
  Equip — Equip [cost] ([cost]: Attach to target creature you control. Equip only as a sorcery.)

• WWizards of the Coast2008-05-01
  Official Ruling
  When you activate an ability, you untap the creature with that ability as a cost. The untap can't be responded to. (The actual ability can be responded to, of course.)
• WWizards of the Coast2008-05-01
  Official Ruling
  If a creature with an ability hasn't been under your control since your most recent turn began, you can't activate that ability, unless the creature has haste.
• WWizards of the Coast2008-05-01
  Official Ruling
  If the permanent is already untapped, you can't activate its ability. That's because you can't pay the "Untap this permanent" cost.

Rulings via Scryfall