From 'Dark Heresy: Apocrypha' (produced by Black Industries); paraphrased:
Mounts- The most basic of "vehicles" is an animal used for riding. While riding on a steed the PC sustitutes his/her Movement for that of the steed. A steed may carry a rider for 8 hours, plus a number of hours equal to its TB; if this duration is exceeded the steed must take a Challenging (+/0) Toughness Test, with failure indicating the animal suffers from Fatigue. Should the steed fail a second Toughness Test the animal collapses from exhaustion. Such overuse of a steed (or cruel treatment) may (at the GM's ruling) injure the animal. Note: I'm paraphrasing, and the preceeding assumes the SB and TB of a mount is high enough to allow it to carry a rider (and any gear). If not, "fudge" something (only if needed, perhaps double the animals SB only for the purposes of Lifting, Carrying, Pulling/Pushing limits). Unnatural Strength (x2) would not be an appropriate "fudge".
Untrained Mounts- On the first Turn of combat, a rider must pass a Difficult (-10) Wrangling Test to control the animal, if this Test is failed the animal/steed/mount flees from the combat at its fastest rate of Movement. This Test is taken each Round until the animal has been brought under control by its rider, and this control lasts for 1 Round plus 1 Round per DoS on the Wrangling Test. At the end of this time the rider must Test again to maintain control of the mount/steed. These Wrangling Tests are Full Actions. Riding an untrained mount into combat confers no "mounted combat" benefits, and the rider/steed suffer a -20 to all Dodge Tests.
Trained Mounts- While fighting from the back of a steed trained for war (specifically, so I'm assuming any Rough Rider mount fits this category), the rider gains the following benefits: +10 WS (High Ground modifier? perhaps negated if fighting a mounted opponent?); use the steed's Movement values rather than your own; may employ the Trample Combat Action; may use a Lance (or similar weapon, such as a boar spear). The rider/steed suffer a -20 to all Dodge Tests. When hit in combat (or striking an enemy in combat) use the following Mounted Hit Locations (assuming the rider is mounted on a Quadruped):
01 Rider Head
02-03 Rider Right Arm
04-05 Rider Left Arm
06-15 Rider Body
16-20 Rider Right Leg
21-25 Rider Left Leg
26-30 Steed Head
31-60 Steed Body
61-70 Steed Right Front Leg
71-80 Steed Left Leg
81-90 Steed Right Rear Leg
91-100 Steed Left Rear Leg
Obviously, if the rider is mounted on a Loxatl (jk) the GM will have to make some adjustments…
New Combat Action: Trample (Full Action)- You direct your steed to run down and trample your opponent (duh). Your opponent may Dodge (if able, aware), and if the Dodge is failed your opponent takes Damage equal to the steed's Natural Attack (it says a horse's Damage is d10+1 [Impact], Primitive; I'd go with max 8 on the d10 just because the horse is so big, it's moving at speed, etc; it also seems to not be following published adventure standards of adding SB to Damage, so…looks like d10+6 [Impact], Primitive [8?]). Perhaps the steed must move a minimum of its Charge Move (18)? In any case, Mono horse shoes ftw.
Horse- Movement: 6/12/18/36, Bestial, Hulking, Quadruped, Sturdy
Skills- Awareness (Per)
Armour (natural)- none
Slain Mounts- It dies (or is killed) and you…
01-20 Narrow Escape- You manage to roll from underneath the steed just in time. Rider is Prone and must take a Stand Action to regain his/her feet.
21-50 Thrown- In your steed's death throes you are thrown, flinging you d5 meters in a random direction; take d10+1 Impact (Falling) Damage (meaning it's reduced by TB but not Armour) to a random Location.
51-90 Partially Trapped- One of your legs becomes trapped under your steed; rider takes d10+4 (steed's TB) Impact Damage to random leg (left or right), rider may not move or use any Reactions until freed (Challenging Strength Test, Full Action).
91-100 Completely Trapped- Your steed falls completely on top of you, crushing you beneath its bulk; take d10+5 (steed's SB) Impact Damage in the first Round the rider is completely trapped, +10+2 in each additional Round until freed (I would say rigid Armours like Feudal Plate and Carapace might protect vs. this Damage, but Leathers, Hides, and other pliable Armours not so much…). To free oneself, the rider (or those coming to his aid) must succeed on a Hard (-20) Strength Test. The rider may benefit from cover while completely trapped, but may not move and cannot use any Reactions (that Uruk Hai is gonna bash your noggin, Gimli!).
Perhaps this will help until an "official" ruling is published.