I remember reading that carriage horses were made to move more "gingerly" by
putting ginger underneath base of tail, where the irritation allegedly made their
movements livelier.

From the Internet:
The other practice involving tails is "gingering" of Arabian horses. In contrast to the western
pleasure horse, the Arabian should be spirited and reactive. Therefore, his tail should be held up.
In order to achieve high tail carriage in phlegmatic horses, ginger is inserted into the anus or rectum.
The stimulation, presumably of pain receptors in the mucosa causes the horse to raise its tail. It
takes more manual pressure, as measured by a pressure gauge such as a
penetrometer, a device for measuring the pressure necessary to penetrate the fruit, but there are both false negative and false positives. The sensation of heat produced by the application of ginger leads to vaso-constriction of the perianal area and consequent cooling of the surface temperature. This can be measured accurately by thermography under constant conditions, but not under field (i.e., show) conditions [17].