It has been supposed also that this name was given to the slave that, during Roman Triumphs, held a laurus crown over the head of the Dux, standing at his backs, but continuously whispering in his ears Memento homo (remember you are (only) a man), in order to avoid that the excess of celebration could lead the celebrated commander to lose his sense of proportions.
The term became common in latter times, indicating any biga driver.