I was, however, but a very brief period employed as a strapper, as from the care and pains which I took in cleaning and watering the horses they soon mended in their jaded, worn-out appearance, for they were regularly fed, because I was regularly sober, a state, I have been informed, in which my predecessor was never in after mid-day.
Source: wiktionary
On large establishments a head-man superintends the strappers, and the general management of the horses.
Source: wiktionary
A boy was called a strapper because when you cleaned and dressed a horse properly, you had a nice flat towel, and you used to hit them on all the muscles.
Source: wiktionary
“Nabla, Nabla,” says he, “them was the true words of Father Luke's when he said them would lend him a strapper would be repaid twofold. Here comes our own strapper back to us in company with a brave lump of a bullockeen. Faith,” says he, “Father Luke, you're the man for my money, and if ever ye want a strapper again, just tip me the word, and I'll go bail ye'll not want one long; and if ye want two, why, I'll steal one for ye.”
Source: wiktionary