You choose the creature type as Crippling Fear resolves, and then creatures not of that type get -3/-3. Once Crippling Fear starts to resolve, players can't respond to the choice or take any actions until Crippling Fear finishes resolving.

You must choose an existing creature type, such as Fish or Artificer. You can't choose card types (e.g., artifact) or supertypes (e.g., snow).

Crippling Fear affects only creatures that aren't of the chosen type that are on the battlefield as it resolves. It won't affect creatures that enter the battlefield later in the turn, regardless of creature type. If a creature changes creature types later in the turn, Crippling Fear won't start or stop affecting that creature.