In Metal Gear Solid:Peace Walker for the PSP, there are a number of Monster Hunter stages you can unlock, but you have to start them in order to unlock them.
To start the Monster Hunter Stages, first read all of Chico’s briefing files. Now go to Extra Ops 29 and on the beach you will encounter a talking cat, which will unlock the first Monster Hunter stage. Subsequent Monster Hunter stages will be unlocked as you complete them.