Specimen 14 is the secret unseen Specimen which appears at the end of the game.
The Specimen 14 appears as a humanoid with an obscured face, who wields a bloodied axe. They are a demonized version of the protagonist.
In the battle between the Boss version of Specimen 9 and the protagonist, there are two endings to the game. Specimen 14 appears as expected in the bad ending. The protagonist shows it's fighting skills, to the point Spooky decides into creating the protagonist into one of her Specimens.
Many consider Specimen 14 as a valid final-Specimen of the game. While others think otherwise.