What do you call a horse farm helper?

Here on the farm, we call our paid helpers farm/stable hands.

Non-paid workers are called volunteer farm/stable hands.

Some horse farms have programs in which people exchange help on the farm for lessons or an education. These are usually called working students.

Answer2:

stable woman or stable man

Answer3: Stable hand, farm hand, stable man/woman, working student, groom, and lad are all terms used for someone who works on a horse farm.