Mounted Combat

If you perform a mounted attack, you are considered to be Unstable, check that out.
To perform such an attack, you get to roll your lowest die pool, between your relevant attack, and Agility & Ride.

Also, you cannot use Aim while your mount is moving, and to reload a projectile weapon while your mount is moving you need to succeed in an Agility + Riding roll, with the Unstable penalty applied to this roll.
Falling:
If either you or your mount are Shaken, suffer a wound, or are Stunned, you must Roll Agility & Riding to keep yourself on your mount, otherwise, you fall to the ground. If your mount was running when you fall, you take 2D4 damage.

You can perform a cool maneuver too, check out Mounted Charge.