If you’re looking to get the Monster secret in Piggy Book 2 Roblox, then follow this guide:
The first thing you will need to do is to unlock the doors. 
Once done, Go for 5 Eye locations.
1st Location
2nd Location 
3rd Location
4th Location 
5th Location 
You will need to click on them to progress further. 
Now click on this Door to Trigger the Easter Egg. 
It’s really Fast and sometimes gets stuck in the Pavement. 
It seems like this is One of the Monsters Related to the Eye. 
If you Touch it, You get Killed. There is no Jumpscare for the Monster. 