From  this site: 

"The octavilla is a plucked instrument used in string orchestras in art music and rondas in popular music during the late 19th and early 20th century.

 
The octavilla has six pairs of strings, with the same tuning and number of frets as a bandurria but tuned one fourth lower. Due to the little [tiro = I *think* this is the distance between frets?] and small size, its sound is an octave higher than a normal guitar.
 
It's believed to descend from the baroque guitar and with bandurria and laúd [lute] it carries the main melody in rondas and rondallas, although when accompanying a choir it can also be used for strumming chords and second voices, as a counterpoint to the solo singing."
 
This video says it mostly fell out of use because of the contralto bandurria.
 
I added https://www.wikidata.org/wiki/Q56253877 for this