When you say that someone’s story is credible, what you are suggesting is that it sounds believable.
When you say that someone made a credible witness you are suggesting that he came across as someone worthy of trust.
Credulous on the other hand, has a negative connotation. Credulity is a person’s willingness or ability to believe that a statement is true, especially on minimal or uncertain evidence. Credulity is not necessarily a belief in something that may be false: the subject of the belief may even be correct, but a credulous person will believe it without good evidence. Thus credulous means “ready to believe especially on slight or uncertain evidence”.
Example
You can trust this information as it is from a credible source.
HR Manager hires anyone only after getting credible back ground screening
The tour company’s offer for participation in an event is nothing but ceremony staged for credulous tourists
Few people are credulous enough to believe such nonsense