A guitar is an item in Evony TKR (The King's Return).
Guitar is one of the collection items in the exhibition for the Art Hall. It is one of four parts in the Stringed Instrument Hall. After the set is completed at three stars, you can trade additional Guitar fragments for gold.
The description of Guitar in Evony TKR says:
Stringed instruments from all over the world. The important exhibits in Stringed Instrument Hall.
Guitar is known to potentially be obtainable from opening Artwork Fragment Chest II, Artwork Fragment Chest III, Artwork Fragment Chest IV and Artwork Fragment Chest V. The links for each item are below:
Note: This list may be incomplete.