Wikipedia describes the guitarron as follows:
“The guitarrón (literally “large guitar” in Spanish, the suffix “-ón” denoting “large”) is a very large, deep-bodied Mexican 6-string acoustic bass played in mariachi bands. Although obviously similar to the guitar, it is not a derivative of that instrument, but was independently developed from the sixteenth-century Spanish bajo de uña. It achieves audibility by its great size, and does not require electric amplification for performances in small venues. The guitarrón is fretless, the strings are heavy gauge, and the action is high, so that quite a bit of left hand strength is required. The guitarrón is played by plucking always double string (one octave between), which is facilitated by the unusual tuning of A D G C E A with the high A lowered an octave putting it just one octave above the low A.
It is eminently capable of standing alone as an unamplified acoustic bass instrument and is an interesting halfway house between the ABG and the double bass as the clips below show:
This clip features vihuela, guitarron and the best mask work since Kendo Nagasaki