Incredible—not believable, but mostly we mean something a little less strong like “amazing” when we use that word. It’s from the Latin, “credo” meaning “I believe.””The magician performed an incredible act or prestidigitation.
Incredulous—refers to the person who’s feeling the disbelief. If you don’t believe something, you are incredulous, and the thing you don’t believe is incredible. “The crowd was incredulous when they heard the politician’s hyperbolic promises.”