Talking to the Radio Times, Moffat stated:
“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.”
“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…”The Day of the Doctor airs worldwide on Saturday 23rd November - a month tomorrow...
[Source: Radio Times]