You always have the Animate Dead spell prepared. When you cast this spell, its casting time is an action instead of 1 minute. You can also create a Spirit using the spell.
Additionally, when you cast Animate Dead, you can reanimate the corpse of any type of Medium or Small creature that isn't Undead, instead of only Humanoids.. The resulting Undead always uses the statistics of a Skeleton, Spirit, or Zombie.