Summary
('Fallen Down A' and 'Fallen Down B', as heard in Fallen Down)
'Fallen Down' is the theme of Toriel.
It only appears in a few songs, all with direct relation to her.
Notably, Fallen Down was orignally composed for the 2012 Earthbound tribute album I Miss You, as discussed here:
On the Origins of Undertale and Deltarune's Leitmotifs