Heh! Wow. Now
this is the kind of response I was hoping for. Ok, let me start from the beginning: My idea originally was just to have the PCs in combat with the monsters. Mounts didn't figure into the equation at all. Then I thought, well, maybe it would make it easier for them to travel to be mounted. PCs are initially 8th level, almost 9th. So maybe the 6th level mount you mention would be more appropriate? I don't even know what that would look like.
Oh, and as for the defenses being all whonky, well, I just started making adjustments out of the blue. I figured good AC because I don't want the PC's mounts to be easily killed by any wandering monsters that happen by or something. Fort is high because, well, they're strong/healthy as a horse? Ref is decent and will lower for similar reasons. (Nimble as a horse(?) and dumb as an ox?) Would you care to show me how the stat block would look if you did it?
Sure... here are two versions, one at level 6 and one at level 10, where it will be a more significant part of each combat. I'm also going to tweak the design to be more, ehh, fey-mount-ish (in my opinion, anyhow).
A few notes: I switched Cha and Wis around; changed the origin to Fey (Immortal means it's from the Astral Sea); dropped its +6 vs. fear (this being pretty celestial-flavored in my opinion); changed it to unaligned to make it more accessible to villainous fey; added leather barding just because; and added a good fey mount ability. Also, I figure, they're fey, why not a talking horse? So I'd let them speak Elven, not just understand it. But as with so much with dm design, it's all a matter of taste.
Anyhow, two versions:
Feywild Charger Level 6 Skirmisher
Large fey beast (mount) XP 250
HP 75; Bloodied 37 Initiative +8
AC 20; Fortitude 21; Reflex 18; Will 16 Perception +5
Speed 8
TRAITS
Fey Hooves
The feywild charger ignores difficult terrain and can move across any solid or liquid surface.
STANDARD ACTIONS
(mbasic) Kick * At Will
Attack: Melee 1 (one creature); +11 vs. AC.
Hit: 2d8+4 damage.
MOVE ACTIONS
Teleporting Charge (mount, teleport) * Encounter
Requirement: The feywild charger must be mounted by a 6th or higher level rider.
Effect: The feywild charger and its rider teleport up to 10 squares and gain combat advantage against adjacent enemies until the end of the feywild charger's next turn.
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Skills Endurance +12, Nature +10
Str 18 Dex 17 Wis 15
Con 19 Int 3 Cha 8
Alignment unaligned Languages Elven
Equipment leather barding
Feywild Charger Level 10 Skirmisher
Large fey beast (mount) XP 250
HP 109; Bloodied 54 Initiative +11
AC 24; Fortitude 25; Reflex 23; Will 20 Perception +8
Speed 8
TRAITS
Fey Hooves
The feywild charger ignores difficult terrain and can move across any solid or liquid surface.
STANDARD ACTIONS
(mbasic) Kick * At Will
Attack: Melee 1 (one creature); +15 vs. AC.
Hit: 3d6+8 damage.
MOVE ACTIONS
Teleporting Charge (mount, teleport) * Encounter
Requirement: The feywild charger must be mounted by a 10th or higher level rider.
Effect: The feywild charger and its rider teleport up to 12 squares and gain combat advantage against adjacent enemies until the end of the feywild charger's next turn.
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Skills Endurance +15, Nature +13
Str 19 Dex 19 Wis 17
Con 21 Int 3 Cha 10
Alignment unaligned Languages Elven
Equipment leather barding
I hope these help!