Here is a starting point for you to design your own prestige class. [edit: I think you can take this class at 8th level at the earliest. 10 ranks in Ride is the true limiting factor for earliest level to take it.]
Rider of Arvoreen
Oliver and Leus approached the cavern's opening carefully, unsure of what horrors may lie within. The bodies of several humanoids littered the outskirts of the cave in various states of decay, and the smell of death hung thick in the air. The light shed from Samryn, Oliver's holy sword revealed a deep tunnel leading into the bowels of the earth. The stench became nearly unbearable, and yet Oliver pressed onward, determined to rid the nearby village of the evil that had been feeding upon them.
Twists and turns lead Oliver deep into the ground, finally opening into a chamber. When the huge beast came within view Oliver muttered a prayer to Arvoreen and spurred Leus onward. The creature stood tall, setting aside the tattered remains of the last intruder, and wiped the blood from its maw, flashing a wicked grin. Even atop Leus, Oliver's proud and noble steed, the creature loomed a full 10 feet higher.
With a burst of speed that the creature could not anticipate Leus darted forward, Oliver thrust Samryn into the demon's ribcage, tearing through its flesh and organs. With its lifeblood spilling to the floor the demon charged after Oliver, thinking to rend through the back of the small rider. Leus, sensing the demon's approach reversed his momentum and swung out to the left, allowing Oliver to narrowly duck the demon's huge claws. Off balance and unable to slow itself the demon took a second hit, the blade slicing through its throat. With a howl of rage and pain the demon crashed to the floor, unable to continue the fight. Arvoreen's champion had won the day.
A Rider of Arvoreen is a cross between a paladin and a cavalier, a specialist in mounted combat. Where a cavalier is a mounted champion representing a noble house, the Rider of Arvoreen is a mounted champion of the church. This class is a natural transition for paladins looking to improve their mounted combat proficiency, but the sacrifice is great.
Spells cease to increase by level, and turning abilities do not increase. This class may be adapted for any deity.
Hit Die: D10
Requirements:
Feats: Weapon Focus (sword or lance), Ride-by Attack
Abilities: Special Mount (as paladin)
Knowledge (Religion): 4 ranks
Ride: 10 ranks
Equipment: Masterwork heavy armor, masterwork sword
Class Skills
Diplomacy (Cha), Intimidate (Cha), Knowledge (Religion) (Int), Profession (Int), Ride (Dex).
Skill Points at Each Level: 2 + Int modifier.
Level BAB Fort Ref Will
1st +1 +2 +0 +2
2nd +2 +3 +0 +3
3rd +3 +3 +1 +3
4th +4 +4 +1 +4
5th +5 +4 +1 +4
6th +6 +5 +2 +5
7th +7 +5 +2 +5
8th +8 +6 +2 +6
9th +9 +6 +3 +6
10th +10 +7 +3 +7
Abilities: With the exception of Divine Blessing, all the Rider's abilities apply
only when mounted on his special mount, as they rely upon the bond between him and his mount.
Level 1 Abilities: Mounted weapon bonus (choose sword or lance - same as weapon focus) +1, Ride bonus +2, Tall in the saddle +1, Divine Blessing.
Level 2 Abilities: Lean into It, sword or lance (while mounted only), Divine Speed.
Level 3 Abilities: Mounted Weapon Bonus sword/lance +2, Tall in the saddle +2.
Level 4 Abilities: Divine Charge 1/day, Ride Bonus +4.
Level 5 Abilities: Mounted Weapon Bonus sword/lance +3, Tall in the saddle +3, Divine Perseverance.
Level 6 Abilities: Divine Charge 2/day, Full Mounted Attack.
Level 7 Abilities: Mounted Weapon Bonus sword/lance +4,Tall in the saddle +4.
Level 8 Abilities: Divine Charge 3/day, Ride bonus +6.
Level 9 Abilities: Mounted Weapon Bonus sword/lance +5.
Level 10 Abilities: Divine Wrath, Ride bonus +8.
Class Features:
Weapon and Armor Proficiency: The Rider of Arvoreen is proficient with all simple and martial weapons, all types of armor, and all shields.
Divine Blessing: Paladins can freely multiclass with the Rider of Arvoreen, and his mount continues to improve based on the combined paladin/Rider of Arvoreen level. The paladin's smite evil and lay on hands ability also continue to improve with each level of this class.
Tall in the Saddle (ex): The Rider of Arvoreen gains a bonus to his ride skill check whenever he uses the mounted combat feat to negate a hit his mount takes in combat. (This bonus is intentionally
unnamed.)
Divine Speed(ex): The Rider of Arvoreen can urge his mount to a bonus speed of +10 to its normal movement rate with a successful ride check vs. DC 20. This bonus lasts only one round, but there is no limit as to how many times it can be done. This is a combat maneuver only and cannot be used to speed overland travel.
Divine Charge(su): When mounted and using the charge action, you deal triple damage with a melee weapon, up to the number of times per day indicated. The ability supercedes the spirited charge feat (which allows you to deal double damage).
Lean into It (ex): The Rider of Arvoreen gains a +2 bonus to damage rolls with his sword or lance. This is similar to weapon specialization, and does not stack with weapon specialization.
Mounted Weapon Bonus(ex): The Rider of Arvoreen gains a bonus to his attack roll when using a sword or lance while mounted. The bonus is to either a sword or lance, depending upon which was chosen at first level, and this weapon must be the same one for which the Rider has Weapon Focus.
Ride Bonus(ex): The Rider of Arvoreen gains a
competence bonus to ride checks.
Divine Perseverance(su): Once per day, if the Rider of Arvoreen is brought to -1 or lower hit points, she automatically is cured of 1d8+5 points of damage.
Full Mounted Attack(ex): At 6th level, the Rider of Arvoreen may take a Full Attack when his mount moves more than 5 feet rather than being limited to one attack. These attacks may be taken against multiple opponents during the move, but the Rider will draw Attacks of Opportunity as usual. This ability does not apply when charging.
Divine Wrath(su): The Rider of Arvoreen channels a portion of his patron's power into wrath, giving him a +3
sacred bonus on attack rolls, damage, and saving throws for a number of rounds equal to his Charisma bonus. During this time, the Rider of Arvoreen also has damage reduction 5/-. This ability can be used once per day and it invoked as a free action.
This class must choose either sword or lance to specialize, and must, of course, have weapon focus in the weapon chosen. This choice affects all special abilities - they are EITHER for sword or lance, not for both.
[edited: a number of tune-ups]
edit: see
http://www.enworld.org/messageboards/showthread.php?s=&threadid=10318 for a House Rules discussion of the Rider of Arvoreen