If you select the check box next to the horse name in horse lists (My Horses, Group details, Paddock details) or in the header of the list, a new menu is displayed with options that apply to all the selected horses.
This is a very helpful menu that saves lots of time not forcing you to enter the information horse by horse.
The full batch actions menu:
Some options allow to enter the information once and it is copied to each selected horse (Farrier, health, lab tests, etc). Other options display a list of selected horses and fields next to each one to enter the information (Field edition, diets, status, measures). And you have some others that are like reports using the selected horses as targets.