As an interim measure, you could consider using the Mount rules from DDM2.0 (
http://www.wizards.com/dnd/files/DDM_ErrataFAQ.zip):
Mounts:
The Mount ability specifies a keyword, such as Dwarf, Orc, or Human, which permits a creature with that
keyword to become its rider. While mounted, both rider and mount are considered to occupy the mount’s
entire space, and are adjacent to one another, but do not provide cover for one another.
The mount and rider are separate creatures which move together. If movement would provoke an
opportunity attack, the attacker would need to choose either the rider or the mount for the target of the
opportunity attack unless the attacker can make more than one opportunity attack during a creature’s
turn.
A mount and rider activate separately.
Mount: On a mount’s turn, it is able to move and attack normally. Effects that move, shift, or change
position of the mount also change the position of the rider. While the mount is moving, you ignore
limitations to the rider's movement, such as those caused by Visejaw, Fear’s Dark Shadow, or standard
conditions such as staggered, slowed, stunned or helpless. If the mount has Flight, the rider also benefits
from Flight while the mount is moving. Other abilities of the mount do not benefit the rider unless
specified.
If the mount is eliminated, the rider is placed on the map in the space formerly occupied by the mount. If
the mount’s former space is not a legal position, place the rider in the nearest legal position.
When the mount charges, if an ability or effect allows the rider to make an attack as part of the mount's
charge, the rider also receives a +1 attack bonus to that attack against the target of the charge.
Rider: While mounted, a rider cannot move on its own but can otherwise act normally. It is still subject to
effects that push, pull, or otherwise transport it. Effects which move, shift, or change position of the rider
do not change the position of the mount. While mounted, a creature’s Speed is 0.
Mounting: To become a rider, a creature adjacent to a mount is removed as a move action that provokes
opportunity attacks. During setup at the start of a game, a player may elect to have a rider already
mounted.
Dismounting: To dismount, on its turn a rider can be placed adjacent to the mount in any legal position
as a move action.
Forced Dismounts: A mounted unit must always consist of an allied mount and its appropriate rider. If
the rider and mount are controlled by different players, even temporarily, dismount the rider (the mount
rejects the rider; place in a legal adjacent space). Similarly, if an inappropriate rider is somehow
transposed with a mount’s rider (a non-Dwarf on a thunder tusk, for example), that creature is also
automatically dismounted. A rider subjected to an attack, ability or special power that pushes, pulls or
slides is dismounted. The rider is considered to occupy the entire space of the mount – so pushing the
rider one space would place it in a space adjacent to the mount. Place effects, such as Abduct or Baleful
Transposition, could also be used to dismount a rider. Forced dismounts do not provoke opportunity
attacks.
DDM2.0 stats for the Warhorse from the Desert of Desolation set:
Warhorse
Level 4
Speed 8
AC 17
HP 30
Attacks: +6/+6 (5)
Human Mount: Can have a Medium or smaller Human as a rider.
Charger +10: When charging with a rider, the rider gets damage +10.
Cheers,
Roger