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 above applies to cases there the rider is simply hanging on to the steed and trying to act. If there is a dedicated saddle and the rider and steed have some sort of communication with each other, than the rider may have an extra die in the combined dice pool. So, if the steed uses all four dice for something, the rider may still have use of one die. If the steed uses two dice to duck and donates his other two dice to the rider, the rider may have three dice to attack. In this case the steed wouldn't be able to donate all four of his dice to the rider as the rider may not use more than four dice in a given turn.
Generally a saddle must be 10% the size of the rider to be effective.
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.
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.