The word was originally rather grander in status, as in bridegroom, and the very socially elevated offices in the English Royal Household of:
The meaning "male servant who attends to horses" is from 1667. The verb is first attested in 1809; the transferred sense of "to tidy (oneself) up" is from 1843; the figuratrive sense of "to prepare a candidate" is from 1887, originally in U.S. politics.
A groom in private service is expected to be 'on call' during specified hours in case any member of the employer's family wishes to ride. Grooms whose employers are involved in horse sports or horse racing are often required to travel with their employers to provide support services during competitions. The services required vary with the type of competition and range from simply ensuring that the horse is ready for the start of the competition to warming the horse up beforehand. In combined driving the groom is the passenger, and at speed is required to shift their weight to balance the carriage.
The historical synonym (H)Ostler has meanwhile got a novel meaning as rail employee. In large establishments there may be several grooms under the management of the head groom or stablemaster. In many cases the head groom has complete responsibility for the horses including devising training schedules, choosing feeds for optimum nutrition and ensuring the horses are shod, wormed, inoculated and provided with timely veterinary care.
Several other words originally denoting other, often much higher titles, notably Constable, Equerry and Marshall, have developed into terms for those working with horses.