A small or medium creature occupies one square and large creatures typically occupy four. Does this mean that a small or medium creature riding a large mount will be out of reach of melee attacks that only have 5 feet of reach ? Does this also mean that such a rider will need a reach weapon to attack enemies adjacent to his/her mount ?
Mounted Combat says
A horse (not a pony) is a Large creature and thus takes up a space 10 feet (2 squares) across. For simplicity, assume that you share your mount’s space during combat.
(Emphasis mine.) In general, this means that a being on a horse will not protect the rider, and that the rider does not require a reach weapon to attack enemies.
This topic is not properly addressed in the core Mounted Combat rules. It was addressed in Errata after it kept coming up in official Pathfinder Society play. Here's how it's done in PFS, after application of errata:
- For both attack and defense you count as occupying all of your mounts squares. Thus you may attack out of any square your mount occupies and foes can attack you into any square your mount occupies. If one square of your mount is hit by a Fireball then you also get hit by the fireball.