Hi. This instructable will teach you to get a pet monster in minecraft ( pocket edition or PC) it should be an under ground room but it does not have to be.
Step 1: Dig an Extra Room
First dig an extra room that comes off of your original underground room. It also helps to remove any torches you might happen to have in that room.
Step 2: Put Glass on It
Need I say more?
Step 3: Cover the Glass
Or you could use any other block that is not see through
Step 4: Wait
Wait for the darkness to begin spawning monsters.
Step 5: Your Done!
Remove the dirt after a few minecraft days and see how many mobs you got!
Step 6: UPDATE
I have recently learned that mobs can de-spawn if you go too far away from them. This can easily be solved by placing dirt over the glass if you are going far away so if they de spawn then new ones will spawn.