This song's tune originates from nineteenth century Warsaw (Poland) and it was used in a lullaby titled "Wlazł kotek na płotek", written by Oskar Kolberg and composed by Stanisław Moniuszko. However the Lyrics for this lullaby were about a kitten on a fence.
The modern song which we know now is in fact named "X". The lyrics were written by Edith Newlin and the tune is traditional. It was published in 1937 by Willis Music Co. These lyrics differ slightly to the one used in the show Y, with the show changing the sequence.
The song is originally called with a slightly different name, and parts of its lyrics sequence have been altered for the version sung in Y. Mentioned in Y at times, X appeared first in S01E11 and continues to appear in each season. When sung to the protagonist, "X" has never been shown to finish without being interrupted and he demanding for it to be sung "from the top", showing his extreme attachment to the song.
In November 2010, Y held a contest where contestants uploaded their version of "X", and other users could vote for their favorite.
Id X and Y.