The OOC idea here is mimicking old-world considerations in horse breeding. Not everyone rode the heavy war horses, even if they could. Scouts had no use for them. Code wise, I'm thinking of a knockoff of Achaean metamorphosis.

Grooming-You know how to provide for a mount's basic needs

(Basically, feeding.)

Mule-Bumble along on a stubborn mule

And under the "mule" skill, the following skills become usable while mounted:
Tenacity-order your mule to hold his ground

(Fear spells have no effect on the mount. Rider isn't going anywhere, either, although the fear affliction remains. Cannot move while activated. 2 second equilibrium to add/remove)

Bray-annoy your opponents to the point of distraction

(Target loses eq for 1 second. 3 second eq loss for user.)

Mounts-see AB RIDING MOUNTS.

Soothing-see AB RIDING SOOTHE

Mare-Prance about on a stylish show horse
Trot-Hurry through town on your swift-footed steed

(You can move three rooms per second before hitting the hasty message)

Jump-Leap the garden gate

(Clear obstructions, 2 sec balance)

Steedtrade-see AB RIDING STEEDTRADE

Whistle-I think this is what the skill is called. The one that summons mounts. I haven't been on my Achaean character in some time, so I forget.

Workhorse-Control the steady work breeds
Stalwart-Your horse is confident with you saddled atop

(A mounted defense that's more for the mount than the rider. Stripped in place of a fear attack against a mount, like quicksilver and aeon. Unlike tenacity, still allows for free movement. EQ time up for grabs. Part of me says it should be slow so as to not make fear attacks useless, since soothing takes a fair amount of eq, but the other part of me is looking at how high up the skill is)

Defense-The size of your horse makes you a difficult target

(Like AB RIDING DEFENSE, with no evasion bonus)

Crash-He'll move if you run at him.

(Run through a person blocking an exit. Unlike jump, does not clear walls. Blocker must reset his block to prevent others from following. 2 sec balance)

Stomp-Bid your enemies to stand away.

(Relatively minor blunt damage. A bonecrusher should never desire to use it, but a mage fighting a lucidian would prefer it, given the balance of weaknesses and resistances.)

Fording-Trod through the rivers of the Basin while mounted

Scouthorse-Dash across the open plains on a swift scouting horse
Trot-See above
Gallop-Your speed is unmatched when you empower your mount

(Costs 2-3 power, no eq. You may move four rooms before hitting the hasty message. Duration of perhaps 3-5 minutes?)

Jump-See above
Fording-See above
Evasion-You can maneuver your agile mount around attacks.

(Like AB RIDING DEFENSE, minus the damage reduction)

Lightwarhorse-Ride your noble stallion into the fray
Fording-See above
Jump-See above
Trot-See above
Stalwart-See above
Evasion-See above, more effective with a higher level mount
Stomp-See above. Minimal damage increase with a higher level mount.
Mountkick-Thrust an unlucky victim out of the way

(AB RIDING MOUNTKICK)

Plow-The valiant knight can mow down his unmounted foes

(Costs 5 power, balance equal to 1.5 times the normal weapon balance. All unmounted fighters in the room are attacked as though a wielded weapon had been swung at them. No deep wounds, accuracy based on the same factors that already determine weapon accuracy. Knights, stop whining, I'm throwing you a bone.)

Heavywarhorse-Sit high above the battle below
Fording
Crash
Defense
Stalwart
Stomp
Mountkick
Plow
Trample-Break the limbs of your prone opponents

(As AB RIDING TRAMPLE, with a 3 power cost)

Blocking-Manuever your horse to block escape

(Block an exit)

Winged-Soar above the Basin on a flying mount
Jump
Evasion
Defense
Fly-Take flight while mounted

(Note that the winged horse, the fabled skill, has few offensive powers. Flight and the combination of evasion and defense are the only compensation. You've really got to need to fly to squat on one of these)

Climbing-Cling to your saddle as your mount scurries in the treetops
Trot
Evasion
Climb-You can guide your mount in the trees
Pounce-Leap upon an unsuspecting victim from above

(Second of three universally practical offensive skills. Victim standing below the attacker in a forest room is dismounted, stunned for 2-3 seconds, left prone, suffers a head wound, and takes a fair amount of damage. Costs 5 power and a 5 second eq)

Legendary-Tame the greatest beasts to be found
Fording
Jump
Barge
Trot
Gallop
Stalwart
Defense
Evasion
Blocking
Mountkick
Plow
Trample
Rampage-Order your mount to unleash a fierce attack on all around you.

(The third offensive riding feat. The mount itself attacks all in the room. Unlike plow, it affects mounted riders. Not practical for group combat, but might be a viable option for the lone hero who gets teamed. Balance should be quite short. The mount attacks, leaving the transcendant rider free to finish the job. Somewhat more damaging than stomp. 10 power, 1 second balance. Note that it can't be used in conjunction with plow, at least before your power recharges.)

If this skill order is kept, the rider would gain one new mount type or two utility skills each skill level. Of course, the mounts would need to be reclassified, and some new ones added to fill holes.

Criticism always welcome.

Edited for consistency in skill names

I like

Nice, but it seems like some of the feats suggested take too much power to be very useful...