The choice of creature type is made as Titan of Littjara enters the battlefield. Players can't respond to this choice. Titan of Littjara's second ability starts applying immediately.

To choose a creature type, you must choose an existing creature type, such as Vampire or Knight. You can't choose multiple creature types, such as "Vampire Knight." Card types such as artifact can't be chosen, nor can subtypes that aren't creature types, such as Jace, Vehicle, or Treasure.