I had a wonderful older TB ex-event horse in my barn (looked a little like Nelson, MIE) and I loved to watch the MT work on him. He loved it, and would grunt, groan and sigh during the massage. He would also lean into the MT's pressure when she found a sore spot. After she did his neck, he would stretch out, do a little Flehman's, and shake his head and neck like a dog. At the end of the massage his head would be almost on the ground, with his lower lip hanging and drooling.
His owner paid for more MT than he probably actually needed (though not a concept I recognize for myself) because the old guy enjoyed it so much.
In contrast, my own horse at the time was one that didn't even like to be groomed, had to be sedated for clipping and the only place he would tolerate massage was his head. He didn't even like the light top of the neck massage I routinely do when warming up.