Inmates

Inmates are villains who are/were imprisoned yet manage to keep some measure of power within their respective prisons and pose a threat to said prisons themselves. This type of antagonist is common in fiction dealing with prisons and goes hand-in-hand with the related gaolers.

This is also the place to put gangs and their members which were similar to street gangs, but worked primarily within prison confines (both fictional and real).

Good examples include most villains from the Batman franchise.

Editing note: Please only include characters who were in a prison and had some degree of power within these confines (Gangs or Crime Lords who functioned mainly behind bars). They are not characters simply imprisoned with no effect on others.