What is a prefix for credible other than in?

There isn't a single, universally accepted prefix for "credible" that conveys the same meaning as "in-" (meaning "not"). However, here are some options depending on the nuance you want to express:

General Negation:

* Un-: "Uncredible" is a common word, though it's often used in a humorous or hyperbolic context.

* Non-: "Non-credible" is more formal and direct.

Doubt or Lack of Trust:

* Dis-: "Discredible" implies something that is damaging to reputation or trustworthiness.

* Mis-: "Miscredible" could suggest something falsely presented as credible.

Other Options:

* In-: While you specifically requested alternatives to "in-," it's worth noting that "incredible" has a slightly different meaning (meaning "unbelievable" or "amazing").

* Pseudo-: "Pseudo-credible" implies something that appears credible but is actually fake or misleading.

The best choice depends on the specific context and the exact meaning you want to convey.