From Nookipedia, the Animal Crossing wiki
Forest Life is a K.K. Slider song introduced in Doubutsu no Mori. It is one of the three songs in Doubutsu no Mori, Doubutsu no Mori+, and Animal Crossing that the K.K. Slider plays whenever the player enters an invalid song, with the other two being "To the Edge" and "My Place". It becomes obtainable in Doubutsu no Mori e+ as one of the new songs in the game. This song is based on the theme of the original Animal Crossing. It is a traveling song with intricate, melodic designs. It has bells/chimes, etc. the chimes make a trickling droplet texture, as a forest will have trickling droplets everywhere from rain on the leaves. Savannah, Skye, Simon, Opal, Peck, and Alfonso have this song in their houses in Animal Crossing: Wild World and later, and Bones has this song in Animal Crossing: New Leaf. Maddie and Maggie both have this song in their homes in Animal Crossing: New Leaf - Welcome amiibo.
It can always be played by K.K. Slider on Saturday between 8pm and midnight next to the Train Station when the player selects "Oh yeah!" while K.K. Slider is asking the player about requests and then entering "Forest Life" as the song. When K.K. Slider has finished playing the song, he will say the same thing as when the player enters in a song request that does not exist, and as a result the player will not obtain anything.
Originally planned as obtainable in Animal Crossing
Because K.K Slider will always play this song whenever the player requests "Forest Life" in Animal Crossing, this could mean that the song was originally planned for the localized Animal Crossing, but later scrapped and implemented in Doubutsu no Mori e+ instead. The coding for the music used on a stereo in Animal Crossing does not exist however.