Cryo Prison-Commonly used by the Griffin Moot and the Zebra Kingdoms, it’s just easier to pull the cortical stack of your really bad criminals, stick them in a virtual environment that can hopefully work towards rehabilitation, and not have to worry about things like escape attempts or prisoners hurting each other. And, since you can run the prison at accelerated speeds, you can turn a fifty year sentence into a year, easily.
The Kingdom of Equestria avoids this-they prefer to use more conventional prisons, restraining bolts, and AI-assisted therapy in real-time to help criminals to find their way to something that is healthier than being evil. Sometimes this works very well, sometimes this just teaches the criminal how to do things better.
The Senate and Minotaur Republic makes the most of prison planets, where criminals have to work towards getting back off the planet by one means or another.