A Mount is a type of Gear equipped in the Mount slot, of which there is one. Mounts are commonly used in combat scenarios as well as some exploration scenarios. If a Mount is equipped, the equipping entity is said to be mounted.
If a Mount and its owner become separated, it is often possible for the owner to call the Mount on their turn; the Mount may then move toward the owner. Whether or not this is possible and the path the Mount takes are determined by the DM on a case-by-case basis.
Generally, Mounts cannot be specifically targeted and slain. This is primarily to avoid the problem caused by permanent loss of Wealth as well as the general frustration that may ensue. Instead, there are various mechanisms to force a target to unequip a Mount, and some Mounts have limited Reactions to emulate their ability to be targeted to an extent within a battle.
The DM may determine that a Mount is lost or slain, however, especially as a result of failing a scenario. Particularly when expensive Mounts are lost in this manner, the DM should offer additional rewards later so the PC can recoup the loss (and perhaps some immediately to reflect the lack of continued upkeep for the lost Mount).
No training is required to equip a Mount.
While a Mount is equipped, two-handed Weapons cannot be used unless otherwise specified.
In combat, the position of Mounts should be tracked even when not equipped. Mounts will follow their owner about whenever possible, moving at the same time as the owning entity; however, any restrictions they confer on movement (such as climbing) also apply when they move about on their own, which can prevent them from following. Mounts can only be equipped if they occupy the same section as their owner. Some Mounts may have special exceptions to these rules.
For more information, see Player's Guide Chapter 7: Gear and Wealth.