Producer Piers Wenger says -
We’re much more interested in new monsters than we are in classic ones, to be honest. I watched an assembly of the first three weeks of filming of the first two episodes on Saturday, and I e-mailed Toby Haynes, who’s directing those episodes, to say 'You are shooting the coolest and possibly the scariest new ‘Doctor Who’ monster in years.' I really, really truly believe that. They are quite terrifying. ”