There are several ways for a monster to respawn:
- Return to the world map.
- Doing so will reset each location and when entering again a new monster set will be present. There are usually two different monster sets available for each location, the one to be used is random (although some sets might depend on other factors like the current quests or how far your are in the story). There is no guarantee that you will get the same monsters (definitely including rare monsters) again when reentering a location.
- Wait some time
- All regular monsters, and some rares, do respawn after some time (about 3 minutes on XBOX, and between 5 and 12 minutes on PC).
- They won't respawn if you're standing too close to their spawn area, however.
- A third option is to move two maps away in the same location.
- All regular monsters will respawn on the other maps.
Please note that some rares have specific requirements (quests, or the first time you enter an area) and can only be fought once in the whole game. Also boss monsters can never be respawned.
The following works only on the XBOX:
- The fourth option is to move one map away, save, return to the title screen and load the game again.
- Doing so will respawn all monsters on any map except the one you are currently in.
The later two options also work for most rare monsters, so you can effectively fight them again multiple times once found.