B is for... Bedspread Monster

When the Doctor wanted Clara to use the telepathic circuits to pilot the TARDIS into her past, her thoughts of Danny Pink got in the way and the ship materialised outside the children's home where he once lived instead. The Doctor was seeking the origins of the popular fear people have of something lurking under their beds. He had developed a theory that some being existed which was so successful at camouflage that no one knew of it - having never been seen. It was the ultimate evolutionary form, since nothing could ever prey on it.
The Doctor and Clara met the young Danny - real name Rupert - and witnessed something appear in his room. When Clara and Rupert climbed under his bed, something entered into the room and sat on it. Looking up they saw a small shape hidden beneath his red knitted bedspread.
They turned away from it, and so did not see what emerged from beneath the covering. It left the room before it could be identified. The Doctor claimed that it could just as easily be one of Rupert's young friends, playing a prank - but neither Clara nor Rupert were convinced by this.
If this was one of the hidden creatures, then it, or another of its species, survived to the end of the universe.

Played by: Kiran Shah. Appearances: Listen (2014).

• From the tiny, out of focus glimpse we get, it does not look like another little boy, so presumably it is one of the Doctor's sought after ultimate evolutionary creatures.
• Like his fellow countryman Deep Roy, for whom he is often mistaken, Shah has a huge number of fantasy credits to his name - having been body doubles for Bilbo and Frodo in the Lord of the Rings & The Hobbit trilogies. He was also the scavenger who tried to steal droid BB-8 in The Force Awakens.
• The "Bedspread Monster" - the only apt title - was on display at the Doctor Who Experience last time I visited - certainly one of the more unusual exhibits.