Nightstalker

The Nightstalker was a being from another dimension which once tried to capture the First Doctor and Susan, on behalf of the Ravenous.

Biology
River Song described the Nightstalker as an energy feeder. It was unable to exist in N-Space without an anchor and it could only emerge at night, as sunlight was like poison to it. It had an armoured carapace which was resistant to photon blasts. (AUDIO: An Unearthly Woman)

Biography
The Nightstalker was recruited by the Ravenous to kidnap the First Doctor and Susan, intending to feed (AUDIO: An Unearthly Woman) on their regeneration energy. (AUDIO: The Odds Against) It claimed that the Ravenous had sensed them, even from the dimension they were trapped in. The Nightstalker arrived in a quarry England in 1963, travelling in a ship which resembled a train carriage. Sheila Page happened upon the carriage while exploring the quarry and the Nightstalker possessed her. Eventually, Sheila began attended Coal Hill School, bringing her close to the First Doctor and Susan.

After seeing the bounty for the Doctor posted in Maldovar's, River Song travelled to 1963, where she encountered the Nightstalker. After the first encounter, the Nightstalker retreated.

The next day, the Nightstalker killed Sheila's foster-carers, Mr and Mrs Mitchell, after Sheila's anger towards them was unleashed. River tracked down the Nightstalker to outside the The Red Lion, discovering that Sheila was acting as a host. River held the Nightstalker down temporarily with an electroplasma net, but this didn't hold for long. Susan then turned up with a dimensional phase battery from her grandfather's TARDIS. River looped the battery into her vortex manipulator, expelling the Nightstalker back into its own dimension, liberating Sheila. (AUDIO: An Unearthly Woman)

Behind the scenes
While the creature is never named in the story itself, in the credits it is dubbed the "Nightstalker".