Eventing ponies (also sometimes referred to as 3 day, or combined training) are trained in multiple disciplines to show off their versatility. The pony must be trained in the following three disciplines:
Eventing, aside from being incredibly fun, also showcases a pony's capability in multiple disciplines and requires endurance on the part of the pony and rider. The training of a eventing pony is essentially split into training individually for each of the three components (usually on separate days) and requires both pony and trainer to seamlessly switch between galloping through muddy trails and jumping logs to performing a polished collected canter.
For shows, the pony and rider are turned out according the typical rules of the particular discipline (e.g. for the dressage portion, the rider is dressed in standard dressage show clothing, and for for cross country pretty much anything goes).
The practicalities of training a human pony to event can make this discipline especially difficult due mostly to the large space requirements for a cross country course (and the time to properly equip the course with obstacles) and the requirement that the trainer run alongside the pony (or trust the pony to complete the course by himself) over a long distance.