JCoxeye's Leitmotif Index:
'Nyeh Heh Heh!'

Summary

'Nyeh Heh Heh!' is the theme of a skeleton that really likes to say "Nyeh Heh Heh!".

('Nyeh Heh Heh! A', as heard in Nyeh Heh Heh! )

Much like 'sans.', 'Dogsong', and 'Lancer', the melody also frequently progresses to a slightly different variation, referred to here as 'Nyeh Heh Heh! B' (as opposed to 'Nyeh Heh Heh! A'):

('Nyeh Heh Heh! B', as heard in Nyeh Heh Heh! )

Together, they make the full 'Nyeh Heh Heh!' leitmotif:

('Nyeh Heh Heh!', as heard in Nyeh Heh Heh! )

As you've likely heard, the motif also has an associated bassline:

('Nyeh Heh Heh! (Bassline)', as heard in Nyeh Heh Heh! )

Finally, as discussed on the page 'On the Origins of Undertale and Deltarune's Leitmotifs', the 'Nyeh Heh Heh!' leitmotif originates from a project prior to Undertale.