Create a Family for your monsters. Do this by selecting 'Families' in the Project bar, right-clicking, and selecting 'Add family...'. A dialog will pop up where you can select your monster, and add it to the list. Now, when you clone the monster, it will create a new monster in the same family. You can apply rules to the whole family, or just single monsters, so they can all - for example - be damaged the same way, but move differently.
You should be able to replace your monster in the current project with a Family in order to create a basic 'class' of monster from its current behaviours by simply using 'Replace object' on these events, and replacing the monster with its family. You may need to create new variables for universal properties like health etc on the family, as instance variables from inidividual monsters will not be visible when referring to their family.