Here is a class that is fairly well-balanced for a better mount/rider relationship and is complete (for 3.0- it needs a tweak or two for 3.5):
Righteous Lance of <Insert Name Here>
A few select holy warriors devote themselves greatly to their mounts and the slaying of evil from atop their steeds. Some of these, with the guidance of their holy orders, and after spending much time in meditation and prayer seeking guidance, become Righteous Lances of <Insert Name Here>. As such, they do not progress in spell casting, but instead gain a few powers that help them in their quest for slaying evil from high atop their holy mounts (low atop their mounts for halfings!). A few fighters or clerics occasionally are granted Righteous Lance of <Insert Name Here> status, though it is most unusual for anyone other than a paladin.
Rules info:
Prerequisites:
BAB: +7
Mounted Combat, Ride-By Attack, Spirited Charge (He must focus on his riding feats)
Ride (7 ranks) (He must ride well)
Religion (6 ranks) (He must be educated in WHY he fights)
Must have patron deity (DM option: Complete a quest for the deity.)
Alignment: LG
Class Skills: The Righteous Lance of <Insert Name Here>'s class skills (and the key ability for each skill) are, Craft (Int), Diplomacy (Cha), Heal (Wis), Knowledge (religion) (Int), Profession (Wis), and Ride (Dex).
Weapon and Armor Proficiency: Righteous Lances of <Insert Name Here> gain no new weapon or armor proficiencies.
Base Level//Attack Bonus//Fort Save//Ref Save//Will Save//Special
1 // +1 // +2 // +0 // +0 // Special Mount, Flexible Charge, Divine Ride I, Ride +1
2 // +2 // +3 // +0 // +0 // Glorious Charge, Ride +2
3 // +3 // +3 // +1 // +1 // Share Divine Grace, Divine Ride II, Ride +3
4 // +4 // +4 // +1 // +1 // Mounted Synergy, Ride +4
5 // +5 // +4 // +1 // +1 // Superior Mount, Divine Ride III, Ride +5
Special Mount: A Righteous Lance of <Insert Name Here> may call a special mount exactly as the paladin. The effective level for this purpose is Righteous Lance of <Insert Name Here> level + 4, unless the Righteous Lance of <Insert Name Here> has one or more levels of paladin, in which case the effective level for determining the mount's abilities is the Righteous Lance of <Insert Name Here> level + Paladin level + 1.
Flexible Charge (Ex): The Righteous Lance of <Insert Name Here> and his mount are blessed with the knowledge of how to charge by an enemy more effectively. When executing a Ride-By Attack, the Righteous Lance only has to maintain a straight line in the portion of the charge that leads up to the attack - after that he may freely change direction. This is restricted only in that each change of direction must be in a generally forward direction (he can make as many turns as he want, but all must be less than 90 degrees).
Divine Ride I/II/III (Su). Divine Ride is a by-product of the divine nature of how the Righteous Lance and his mount act together as a single unit. At 1st level, the Righteous Lance may substitute a ride check for a concentration check when casting spells while the mount is moving (not running/galloping). At 3rd level, this substitution may be made while the mount is running/galloping. Finally, after achieving the ultimate bonding of mount and rider, the Righteous Lance may substitute a ride check for the concentration check anytime he is mounted on his special mount. This ability is always active (it does not take a standard action to activate)
Ride +1 (Su). A Righteous Lance of <Insert Name Here> is granted a circumstance bonus of +1 to their riding skill. This increases to +2 at 2nd level, +3 at 3rd level, etc. This bonus is restricted to when riding the special mount. This ability is always active (it does not take a standard action to activate)
Glorious Charge(Su): The Righteous Lance of <Insert Name Here> may, when charging, mounted on his special mount, and fighting evil, deal a sacred bonus of +2 damage. This ability is always active (it does not take a standard action to activate)
Share Divine Grace(Su): The Righteous Lance and his mount are beginning to merge together into a single fighting unit. The mount now benefits from the rider's Divine Grace (Charisma bonus to saving throws). If the rider did not already have Divine Grace, it is granted now, but for the mount only. This ability is always active (it does not take a standard action to activate)
Mounted Synergy: The Righteous Lance of <Insert Name Here> may take the Mounted Synergy feat (this is not a free feat). The feat is:
Mounted Synergy. (preq - Righteous Lance of <Insert Name Here> level 4, special mount as per Paladin): This character gains a +2 bonus to damage when mounted on his special mount, as he has learned how to take maximum advantage of working together with his special mount. (This bonus is "unnamed.")
Superior Mount: This power is the holiest of the Righteous Lance of <Insert Name Here> granted abilities. His mount becomes celestial, getting the celestial template applied at its current hit die (and re-calculated as its hit die increase, of course). If the mount is already celestial, it does not gain some other benefit instead - the character was simply already blessed. In addition, the connection between the mount and the paladin also intensifies so that they act like a single unit: when mounted, the mount may now benefit from Righteous Lance of <Insert Name Here>'s ability score modifiers to saving throws, using either the Righteous Lance's or the mounts, whichever is higher. This latter beneift is supernatural.
Multiclassing: Righteous Lance of <Insert Name Here> levels combine with paladin levels for all paladin class abilities except spell casting (which the Righteous Lance of <Insert Name Here> does not get). The paladin's special mount, however, is granted a rapid progression such that the effective level for the mount abilities is the Righteous Lance of <Insert Name Here> level + Paladin level + 1. As the Righteous Lance of <Insert Name Here> is divinely selected, he may not freely multiclass. If he advances in any class other than Righteous Lance of <Insert Name Here> or paladin, he may no longer advance as a Righteous Lance of <Insert Name Here>. The Righteous Lance of <Insert Name Here> may multiclass with the paladin class with no restrictions.