What is the etymology of 放我一马？
It should be 放我一马. 马 here refers to the rider rather than the horse itself. It's from 三国演义.


  出自三国演义。赤壁之战，曹操大败，欲从华容道逃走，被关羽逮个正着，立马堵住去路。关羽念及往日旧恩，让开马位，使曹操得以逃脱。“放你一马”，比喻手下留情。即出于此。


Source: http://zhidao.baidu.com/question/2156078.html

English version of the story (wikipedia)