I don't know about a 'cowgirl', but it sounds like you're talking about the song recommended for the Master Set process;
The Set Up Recording:
Ballad of a Runaway Horse by Jennifer Warnes:
You can find this song on Jennifer Warnes – Famous Blue Raincoat, 20th Anniversary Edition, or Rob Wasserman – Duets, or Trios
This song works because of its simplicity and the steadiness of the voice line and the bass line, especially the bass line.
Other recordings could be used provided that they have a simple steady bass line that is easy to hear. I’ve found the above song to work best.