3500 BCE - PRESENT
Easily the most recognizable and contemporary predecessor to the guitar, the harp has existed in various shapes and sizes throughout the ages, all the way from antiquity to present day. In order to be considered a harp, one must feature a neck, a resonator, and strings.
They originated in east Asia; the oldest known harp was found in Sumer from around 3500 BCE.
The Harp's influence on the guitar is most plainly exhibited in European harps; they favored the "pillar", a third structural member to support the far ends of the arch and soundbox, which is still the iconic shape we know today.
Further along, by the Baroque period in Italy and Spain, more strings were added to the harp to allow for chromatic notes and more complex harmonization.
Fun fact: the harp's namesake is often added to instruments outside of the harp family including the aeolian harp, autoharp, harpsichord, harp guitar and harp lute.