Reimu and Marisa's song
Summary by Wrathkal
A former pet yukkuri couple with their only remaining Koreimu are begging to be taken in, but the Koreimu has a shitty mouth, so most of the humans are ignoring them.
Except for one. He tells them he'll take them in if they help him to take it easy with a song. They sing their own tune, but he tells them he doesn't want to hear that, but a song of his own. They agree, and he starts singing.
It's an insulting song about Marisa (in Japanese), saying that the Ma, Ri and Sa are the same characters that are used in a bunch of degrading words. The yukkuris don't sing; how could they? Marisa can't sing something that is purposely uneasy, and Reimu can't bring herself to insult her mate.
But the one who sings it is the Koreimu, whose expression reveals her contempt for her parents for not being able to sing a song. When she demands to be taken in as a pet, her parents quickly seize on it to avoid singing that uneasy song.
But Anon isn't so easily convinced, he tells them there's another half of the song, and this time it's Reimu's version, and this time the Reimus can't bring themselves to sing. That's when the Koreimu starts insulting Anon.
So Anon steps on the Koreimu, torturing it by pressing down to make it throw up paste, telling the parents to sing or else... After a bit of torturing, they finally give in, but beg Anon to let the little one free first, to which he agrees.
But it stupidly insults him again, and this time he steps harder, until paste comes out of its anyaru. The parents finally break into a desperate wailing chorus, though it pierces their hearts with a undeniable truth.
But they have sung, and Anon removes his foot from the child. However it's already too late, the injury is critical. The parents beg Anon to let their child take it easy, but he reminds them that he would keep his side of the promise only if they managed to let him take it easy, which they didn't. So he leaves.
The parents quickly turn to comfort their child, but realize that it's already dead. Their last remaining child, already dead.
They start singing the uneasy song again, no longer wailing, for their child's dead corpse.
Summary of the song by konakona
"The 'M' of Marisa stands for manuke (stupidity), the 'ri' stands for gokiburi (cockroach), the 's' of Marisa is the lowest 's' around (saitei)."
"Reimu's 'R' stands for 'reitei' (low-grade/inferior), the 'i' of Reimu stands for 'kimoI' (gross), and the mu of Reimu stands for 'munou' (useless, no talent)."