How are they going to introduce all these new species without it being a plot hole? Do they expect people to believe that they lived in the shadows undetected for 1000-3000+ years? To explain why people from Originals to Cade never seemed to have encountered them? Or were they intrapped somewhere thousands of years ago and have recently been set free? They implied some go to the school, so them being so ancient would contradict Julie's statement about no more ancient guys with teen girls and so on, unless after being set free they started turning others and creating more of themselves and the school ones are the new ones?
So were they,
▪In hiding because they were afraid of Cade or Sirens or so on?
▪Hunted to near extinction and now only a couple of each left so they stayed secret to stay safe?
▪Or were they so powerful and deadly that they needed to be locked up because no one could destroy them?