The concept of a steed covers any situation where one being is riding on another being. This could be a familiar case such as when a Human is riding an Equine, or a somewhat more unusual case where five Takolee's are riding a Groten. In either case, there are certain issues regarding combat which must be explained.
In most cases, the rider and steed function as a single unit. In a base situation, the rider has no dice, while the steed has the usual four dice to distribute amongst attacking, ducking, movement, etc. However, the steed can transfer his dice to the rider. For example, the steed can duck with two dice and the rider can attack with two dice. If the steed ducks with all four dice, the rider has no dice to use. The steed can steady himself, using no dice to do anything, giving the rider the opportunity to use all four dice for an attack.
In all cases, the steed will be the one ducking any attacks. In the case of a hand to hand defense, either the steed or the rider can parry. And of course, either the rider, or the steed, or both may attack.
The actions of the rider on the steed follow the same rules as other turrets. Namely that an extra die (D6) is available to the cumulative dice pool if the rider either has two stages of an occupation they intend to employ, or the rider is seasoned (>= 5th stage). As a reminder, no more than 4 dice can be used in any single action, either attack or defence.
And as per the dice pooling rules, if the steed uses more than two dice for an action - including donating dice to the rider, then the next instant will have the dice available set to be four minus the dice used.
When attacking a steed/rider, a placement of 4 will hit the rider for general damage (no placement). If you beat the defense by 10 or roll a six for placement, you can choose to either hit the steed or rider. You can also choose placement on the rider - allowing double damage, mortal wounds, and assassinations.