In each of the Animal Crossing series games, K.K. Slider has a handful of secret songs that can only be obtained by request; he will not play them randomly, only if they are given to him by name. They are still part of his setlist, meaning that obtaining them is necessary for completing the catalog. The intended way to obtain these songs is to hear about them from other players, hear them in certain villagers' homes, or on the radio. Villagers will also occasionally tell the player about them.
When a secret song is requested, K.K. will express surprise before playing it. After this, he will give the player a bootleg recording, as with standard songs. Each game typically introduces three secret songs.
Additionally, certain songs cannot be obtained at all, only being heard when the player requests a song that is not available in K.K.'s setlist. These are most commonly found when misspelling a genuine song's name, such as "K.K. Cruising" rather than "K.K. Cruisin'". After hearing one of these songs, K.K. will say that he simply improvised a tune on the spot; therefore, he doesn't give the player a bootleg. In each game, there are three of these "invalid" songs, and they become properly obtainable secret songs in the following installment.
In Doubutsu no Mori, Doubutsu no Mori+, and Animal Crossing
Since this was the first iteration of the series, there are three obtainable secret songs and three unobtainable "not my bag" songs.
One of the unobtainable songs, "Forest Life", is notable for being a remix of the game's main theme.
In Doubutsu no Mori e+ and Wild World
In Doubutsu no Mori e+, the previously unobtainable songs ("My Place", "Forest Life", and "To the Edge") are made available only by request. However, the three secret songs from prior versions still remain request-only, meaning the game uniquely has six secret songs. In Animal Crossing: Wild World, while no new songs are added, "K.K. Song", "I Love You" and "Two Days Ago" are made normally obtainable, setting the trend for the rest of the series.
Three new unobtainable songs are introduced in place of the now-obtainable ones. One of these songs, later named "Spring Blossoms", is a remix of the original Cherry Blossom Festival from Doubutsu no Mori and Doubutsu no Mori+.
Introduced in Doubutsu no Mori e+ is a non-traditional secret song, "K.K. Birthday", which cannot be obtained nor requested. In Doubutsu no Mori e+, K.K. will perform this on the player's birthday if another player gives K.K. a birthday message. It only appears as background music in in Wild World and City Folk, but returns and is obtainable in New Leaf, where K.K. will perform it on the closest Saturday prior to the player's birthday (or the actual day, if it naturally falls on a Saturday). In New Horizons, K.K. will perform "K.K. Birthday" on the player's actual birthday instead, but it otherwise remains the same.
In City Folk
The three unobtainable songs are now available as secret songs, being named "Stale Cupcakes". "Spring Blossoms" and "Wandering". K.K. can play these without being requested through the new "encore" feature. Three new "invalid request" songs are introduced; one of them, "K.K. Island" in New Leaf, is a remix of the Animal Island theme from Animal Crossing.
In New Leaf
As before, the previous unobtainable songs—now named "Hypno K.K.", "K.K. Stroll", and "K.K. Island" —are available as secret songs. Three new ones are introduced. One, named "Animal City" in New Horizons, is a remix of the city theme from City Folk. As this game introduces album art, the secret songs now have art associated with them. They are also reorderable from the catalogue.
In New Horizons
The three unobtainable songs from New Leaf are named "Animal City", "Drivin'", and "Farewell", and are properly available as secret songs. (Unlike in New Leaf, neither they nor "K.K. Birthday" are available to order from the catalogue anymore.) Three new invalid songs take the secret ones' place, as always; on the official Atsumare Dōbutsu no Mori: Original Soundtrack 2 and Atsumare Dōbutsu no Mori: Totakeke Music Instrumental Selection albums, they are given the name "はずれ" (hazure), meaning "miss" or "failure".
- "Hazure01" is a remix of the main theme from Wild World and City Folk, using K.K. Slider's guitar and vocals. It has been extended to play twice, with a bridge in between the two loops.
- "Hazure02" is a funky song with lighthearted major key chords and a breezy melody. It sounds reminiscent of the city pop genre, a form of Japanese funk which was popular in the 1980s, and particularly sounds close to Tatsuro Yamashita's 1982 single, "Sparkle". An official remix of this song is available on the Totakeke Music Instrumental Selection album, under the name "Hazure02 -City & Pop Ver.-".
- "Hazure03" has a bouncy bassline and a nostalgic melody with a focus on the downbeats. It sounds reminiscent of doo-wop music from the 1950s.
- The secret songs generally follow a pattern of having one remixed song in each group of three; "Forest Life", "Spring Blossoms", "K.K. Island", "Animal City", and "Hazure01" are all remixes of themes from prior games.
- Another pattern the secret songs follow is that most do not have "K.K." in their names. The exceptions are "K.K. Song", "K.K. Birthday", "K.K. Island", "K.K. Stroll", and "Hypno K.K."; however, the latter four do not have "K.K." in their Japanese names, maintaining the pattern.
- "To the Edge", "My Place", and "Forest Life" were included on Doubutsu no Mori: Totakeke Music 2 before they became proper songs, meaning that their Japanese names were revealed before appearing in-game. Likewise, "Stale Cupcakes" was included on Gekijōban Doubutsu no Mori: Original Soundtrack before it became a proper song in City Folk.
- In City Folk and New Leaf, the internal names for the invalid songs were included in the game files, meaning the actual names (in Japanese) could be determined. In New Horizons, the invalid songs' titles are concealed and only referred to as "Hazure", even on the official soundtracks.
- If an invalid song is put in a music player via save editing, the title will default to "けけロックとゆかいななかまたち", meaning "K.K. Rock and Happy Friends."