This is used in Gaucho rodeos. The "cincha" is the girth or cinch (US), and the "encimera" is the pad for the rider to sit on. Bareback because no saddle is used, it's for extra stability while still like riding bareback.

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Note added at 33 mins (2017-04-06 18:53:00 GMT)
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"La jineteada gaucha1 es una actividad ecuestre característica y tradicional de Argentina, Paraguay, Uruguay, y sur de Brasil (Río Grande del Sur) que integra la cultura folclórica con estirpe de gaucho de estos países, en particular la cultura gauchesca. Es también tradicional en la zona sur y austral de Chile. La actividad consiste en que el jinete debe sostenerse por entre 6 y 15 segundos sobre un potro (bagual o pingo). Se realiza en varias categorías: crina limpia o potro pela(d)o, grupa sureña o cuero, bastos, con encimera, sin boleadoras, silla, etc., o combinaciones."
https://es.wikipedia.org/wiki/Jineteada_gaucha

"According to the competition, there are several categories. The most important are:

Crina Category or bareback: Rider is only allowed a leather strap, which is placed around the horse's neck. Spurs are used and the mounting time is 8 seconds.
Category surera: Rider must ride the animal with only a sheepskin as a pad. Reins are held in one hand, and in other is whip. The mounting time is 12 seconds.
Category Basto with counter: Rider uses stirrups and should not lose them at any time. The mounting time is 15 seconds."
https://en.wikipedia.org/wiki/Jineteada_gaucha

Bareback Pad - Rodeo Hard
www.rodeohard.com › Bull Riding Gear
Bareback Pad Heavy 1" felt pro rodeo bareback riding pad by Saddle Barn® is covered with roughout leather, skirting leather wear plates.


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Note added at 42 mins (2017-04-06 19:01:39 GMT)
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The fact that the pad has a girth attached hints that it is for rodeo use; in normal, everday riding, the girth holds the saddle in place, not he saddle pad, which is placed underneath.

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Note added at 3 hrs (2017-04-06 21:44:29 GMT)
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This piece of tack, when used in Jineteada Gaucha, is the equivalent to a bareback riding pad used in Western rodeos. When it is referring to the typical Gaucho tack, as in Martin Fierro, "And there the smart gaucho, as soon as he bridled the colt, put on the leather pads and got on him right away", it is just a pad. As this is a piece by an artist, another possibility is to call it a Gaucho saddle pad, or something on these lines.

Cincha and encimera are two parts of equipment tied together with leather straps. The cincha goes under the belly of the horse and the encimera goes over the saddle and beneath the saddlecloth and holds the saddle in place. The sobrecincha or cinchón is a smaller girth that holds the saddlecloth in place.
I am not very sure about the translation of encimera (that is why my confidence level is low), but I found this reference: http://www.arandu.com.ar/productos.php?categoria=93

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Note added at 4 horas (2017-04-06 22:20:30 GMT)
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I found this picture (not a very good one I'm afraid) but it shows the cincha (the part made with strings), the encimera (made of leather) and the correones (leather straps that hold together the cincha or barriguera and the encimera). http://www.folkloredelnorte.com.ar/cinchas.htm

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Note added at 4 horas (2017-04-06 22:51:33 GMT)
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Both parts are usually referred as "cincha" so if you need a title for an art work, I think "Cinch" or "Girth" would work.

I posted an answer before, and all the references are now in the notes section. That answer was too technical for the purpose, though it does explain quite well what an encimera is (the sticking point). Since the title of a work of an art merely needs to be understood and evocative, I suggest just going with saddle, since it is one of the elements of that elaborate setup.

See notes below if interested, and take a look at the Google image for encimera + caballo (or gaucho)- the one with all the labeled parts.