From Nookipedia, the Animal Crossing wiki
The Guitar Set, is a collection of five musical instruments in the Animal Crossing series that made its debut in Doubutsu no Mori. Originally consisting of just three items, the Metal Guitar was added in Animal Crossing: Wild World and the Electric Guitar was added in Animal Crossing: New Leaf. While the Folk and Country Guitars are both based on classic dreadnought-shaped acoustic guitars, the Rock Guitar is based off the real-life Fender Stratocaster, and the Electric Guitar is similar to the Gibson ES-335.
Golden Axe trading event
In Animal Crossing: Wild World, the Golden Axe trading event involves trading multiple items to various special visitors. The Country Guitar can be obtained by talking to Wendell and giving him a Red Turnip, but he may also give the player a Turban, which allows the player to complete the trading event and obtain a Golden Axe. The Country Guitar is traded to K.K. Slider, who will give the player his picture, a rare find indeed. This effectively ends the trading event, but will not reward the player with a Golden Axe.
Doubutsu no Mori, Doubutsu no Mori+, Animal Crossing, and Doubutsu no Mori e+
Animal Crossing: Wild World
Animal Crossing: City Folk
Animal Crossing: New Leaf