Ravagers stables

What are the ravagers? Were do they come from?  You don't find them in outposts or in woodland mansions.  So when a raid starts were do they come from? That's why they should add the ravagers stables. They would have 1watch tower to spy enemy's  and would have 3 to7 ravagers in side  it would also have loot to from stuff to saddle s and emralds