Steven Moffat has come out to explain his reasoning to bring back the Zygons in the 50th anniversary special.
Moffat told Radio Times:
“Every year since I took over [Doctor Who] I’ve been trying to get the Zygons in and then I thought ‘Well, it’s the 50th…’
The Zygons are beautifully designed monsters, they are so wonderful… We barely changed the design at all because it was so good.”
He also added:
“Obviously the Daleks had to be there, the Daleks had to come to the party because they’re really scary and are part of the iconography of Doctor Who – possibly the central part – so we had to have them.
”And in a way it was fun to bring back Doctor Who’s biggest monster success ever, the Daleks, and then go to maybe the smallest monster success [the Zygons] – they were only in it once but everyone remembers them – great monsters, great outfits… and they have these nice whispery voices…”