The writer assured fans of the BBC show on Twitter that the latest nemesis for Matt Smith's Time Lord will be "a good one".
"See this new monster I've invented? This is a GOOD one," he wrote. "This will SCARE you. Oh yes."
Among the villains already confirmed for Doctor Who's upcoming seventh series are the Daleks, who were last seen on the show in 2010, and the Weeping Angels, who will somehow be involved with the departures of Amy Pond (Karen Gillan) and Rory Williams (Arthur Darvill).
The first pictures of Smith with the Doctor's new companion, played by Jenna-Louise Coleman, emerged earlier this month.
Moffat recently insisted that there is "no chance" of the BBC "giving up" on Doctor Who, describing the show as "a huge money-spinner".
Doctor Who will return to BBC One in the autumn, with the series seven premiere debuting at this year's Edinburgh International Television Festival.
Watch the trailer for the next series of Doctor Who below: