Onslaught

• WWizards of the Coast2004-10-04
  Official Ruling
  The ability is not optional. If you have the only targetable creatures, you have to tap one of your own.
• WWizards of the Coast2004-10-04
  Official Ruling
  Can target an already tapped creature.
• WWizards of the Coast2008-04-01
  Official Ruling
  A “creature spell” is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.

Rulings via Scryfall