I'm sure I've seen it somewhere...there is a suggested penalty for dismounting rapidly....
It's called falling
What I'd do is first envisage what sort of movement it is ...
Type 1 - Circus Trick ...
Very Difficult Riding mnv to go from seated to standing on Horse's back. no benefit/bonus from saddle
Tumbling/Acrobatics mnv to flip off the horse (and control where you land and what way you are facing ... "leaping" could be used to land safely, but the GM determines facing). Difficulty based on speed, terrain and distractions (are you being shot at?)
- Mnv ends with character Standing, facing the correct direction (failure => character takes medium falling damage, OB determined by speed & terrain)
Type 2 - Cossack Dismount (also Amer.Ind/Berber/etc)
* Very difficult riding mnv to slide sideways such that you have only one leg over the back of the mount
* Tumbling mnv to drop and roll away from mount in a controlled manner. Difficulty based on speed, terrain and distractions (are you being shot at?)
- Mnv ends with character lying prone (and hopefully unseen by any observer); (failure => character takes small falling damage; OB determined by speed & terrain)