Skittering Skirge

• WWizards of the Coast2004-10-04
  Official Ruling
  The triggered ability triggers when the creature spell is cast, and it is put on the stack before any responses can be cast or activated. This ability will resolve before that creature enters.
• 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