Final answer:
In 'The Binding of Isaac', the familiar that leaves blood creep on the floor is the 'Headless Baby'. This familiar helps by damaging enemies that contact the blood creep.
Step-by-step explanation:
The familiar in The Binding of Isaac that follows Isaac and leaves a trail of blood creep on the floor is Headless Baby. This blood creep causes damage to enemies that walk over it, providing an advantage by creating an area of effect that harms enemies. Other familiars mentioned, like Ghost Baby, Little C.H.A.D., and Demon Baby, have different abilities that support the player in other ways.