The guitar was a popular Human stringed instrument. Nyota Uhura often played the guitar to entertain the USS Enterprise crew.
In 2267, she slowly ran out of strings in her attempt to ward off Space cafard. (TOS novel: Mission to Horatius)
According to Sethan, the ka'athyra was nothing like the guitar, despite some people making the comparison. (TOS novel: Dwellers in the Crucible)
Katalya Tremain played the guitar. (TOS novel: Vulcan!)