As the title says, one of Toby's previous songs, Nightmare Knight, should be on the leitmotif page as a large portion of the song was clearly repurposed for black knife
The others didn't want to include song origins that were outside of UTDR.
but, the leitmotif page also includes songs from outside of UTDR, like jingle bells
"The Door" motif may be derived from Nightmare Knight, but it diverges too significantly and doesn't have a strong enough narrative connection (yes, even including its use in Black Knife) to make a provable reference. As it stands, The Door is an original motif and is documented as such.