1. Difference #1: Humanness.
    Centaurs are part man, part horse. Unicorns- all animal.
  2. Similarity #1: mythological.
    Contrary to popular belief, unicorns are not extinct. They are non-existent. Much like centaurs.
  3. Difference #2: creepiness
    Unicorns are widely accepted as adorable or admirable. Centaurs, on the other hand, generally elicit fear or disgust.
  4. Similarity #2: hooves.
    Both creatures have four hooves. (Centaurs also have two arms though).