Here's a first draft of the PrC. In the first few levels, he gains some combat bonuses while mounted, while I decided his abilities later focus a bit more on leading his men in battle.
Rider of Rohan
The Riders of Rohan form Middle-Earth’s most feared light cavalry, wielding their signature weapon, the spear, with great skill. Hailing from the vast plains of the Rohirrim, the Horse-Men, they have lived together with their horses from childbirth on. None can hope to recreate the bond the Rohirrim share with their horses, not even the immortal elves. Riders of Rohan usually hail from the Fighter class, though Paladins, already having a unique mount, fit the concept rather well, too, as do Rangers and Barbarians with their more wilderness oriented set of skills. Other classes rarely become Riders of Rohan.
NPC Riders of Rohan form the elite core of the Rohirrim Army, and the king’s personal guard (called the Eorlingas [I think, correct me, if you will

])
Prerequisites
Base Attack Bonus: +5
Skills: Ride, 8 ranks, Handle Animal 4 ranks
Feats: Mounted Combat, and Ride-By Attack or Mounted Archery
Special: Must own a light horse and a light warhorse of Rohirrim ancestry, and must be accepted into the Riders of Rohan, an honor usually reserved to denizens of that country.
Class Features
Hit Die: d10
Class Skills: Animal Empathy (Horses only) (Cha), Balance (Dex), Handle Animal (Cha), Listen (Wis), Profession (Wis), Ride (Dex), Spot (Wis), Wilderness Lore (Wis).
Skill Points each level: 2 + Int modifier
Special Abilities
Riders of Rohan are proficient with the Shortspear, the Javelin, the heavy Lance, and the composite Longbow, as well as with light Armor.
Light Cavalry: All class features, except Leadership, Inspired Charge and Ringbearer funcion only if the Rider and his mount are wearing no armor heavier than light.
Ride Bonus: At 1st level, the Rider gets as +2 inherent bonus to his Ride skill. This bonus increases by 2 at 4th, 7th and 10th level.
Fearless Charge: Beginning at 1st level, while performing a charge action while mounted, he gains a +2 morale Bonus to Will saves to avoid mind-influencing effects. This bonus increases to +3 at 5th level, and +4 at 10th level.
Wall of Shields: Beginning at 3rd level, the rider, when dismounted and equipped with at lest a large shield, he may together with at least 4 other Riders (which may be of lower levels) form a wall of shields. The armor bonus of the shields increases by two for all participients, as long as all of them stand in a line and do not move. One level 3 rider is required for all riders below that level, and all participients must be at least 1st level Riders of Rohan.
Skillful Rider: Beginning at 2rd level, the Rider applies his ride bonus (the class ability) to his Initiative checks, and at 8th level as a dodge Bonus to his AC while mounted.
Leadership: At 6th level, the Rider of Rohan receives the Leadership feat for free. If he has this feat already, or acquires it later by normal means, his Leadership score is increased by 2.
Inspiring Charge: If the first action a Rider of Rohan of at least 6th level is a mounted charge (after moving close enough to the enemy), all allies within 30 feat gain a +1 morale bonus to initiative, attacks and damage for a number of rounds equal to his Charisma modifier.
Ringbearer: At 9th level, as a token of his status, the Rider of Rohan receives a ring from his king. This ring is (usually) not magical, but merely a symbol of the status he has attained. A Ringbearer has the right to command his own company of riders in battle, and his special status gives him a +2 bonus to all Charisma-related checks when dealing with Rohirrim, as well as a +2 bonus to his Leadership score for purposes of attaining followers.
Level BAB Fort Ref Will Special
1 +1 +2 +2 +0 Ride +2, Fearless Charge +2
2 +2 +3 +3 +0 Skillful Rider: Initiative
3 +3 +3 +3 +1 Wall of Shields
4 +4 +4 +4 +1 Ride Bonus +4
5 +5 +4 +4 +1 Fearless Charge +3, Leaderhip
6 +6 +5 +5 +2 Inspiring Charge
7 +7 +5 +5 +2 Ride Bonus +6
8 +8 +6 +6 +2 Skillful Rider: AC
9 +9 +6 +6 +3 Ringbearer
10 +10 +7 +7 +3 Fearless Charge +4, Ride Bonus +8
... Well, it's certainly not underpowered.

I think

Hmmm ... what's missing is an ability that improves the horse itself...