The Janus were a humanoid species with two faces.
The Janus looked like humans, save for an identical second face on the backs of their heads. Female Janus could see the future with their front faces and the past with their back faces, whilst male Janus could not; this meant that female Janus were often hunted by slavers. The Janus' ability to see through time was imperfect; events which involved time travellers, or beings with extremely long lifespans, were difficult to see. (TV: Face the Raven)
In the 21st century, two Janus took refuge in the trap street run by Mayor Me after escaping slavery. Anah disguised her daughter, Anahson, as a boy, so that she wouldn't be targeted for her abilities. When Mayor Me needed to set a trap for the Twelfth Doctor, she faked the murder of Anah and framed Rigsy for the crime. The Doctor was able to solve the mystery with the help of Anahson. (TV: Face the Raven)