The Knackers Yard is an amalgamation of the darker side of the Doctor's nature taken from somewhere between his twelfth and final incarnations, ie. Handy.
The Junkyard appears throughout The Trial of a Pie Lord and tries to steal the Sixth Doctor's remaining regenerations.
Characters that could be The Valeyard Edit
As we all know, The Valeyard will someday come back (yes he will, we just have to keep asking
Moffat Chibnall about him and The Rani).
Here is a list of characters that could possibly be The Valeyard in disguise:
Clara (because she never existed)
Whatever the fuck The Silence were
The 1st Doctor
The 2nd Doctor
The Splink Doctor
The Overrated Doctor
The Underrated Doctor
The Pie Lord Doctor
The "IF WE FIGHT LIKE ANIMALS" Doctor
The Based Doctor
The Even More Based Doctor
The Even More Overrated Doctor
The Foot Face Doctor
The Transgender Ginger Doctor
The GOAT Doctor
The Canon Doctor
The Dream Lord