I'm ok with a slightly higher AC. Unicorns at the same size and only 4HD have AC 18.
That'd be my preference to.
How's this for a start on the stats. I've tossed in powerful charge and trample, since the classical sources saying they "kill many men and horses" if forced to fight made me want to make them tough customers:
Monoceros
Large Magical Beast
Hit Dice: 6d10+30 (63 hp)
Initiative: +2
Speed: 50 ft. (10 squares)
Armor Class: 18 (–1 size, +2 Dex, +7 natural), touch 11, flat-footed 16
Base Attack/Grapple: +6/+16
Attack: Horn +14 melee (1d10+9)
Full Attack: Horn +14 melee (1d10+9) and 2 hooves +6 melee (1d8+3)
Space/Reach: 10 ft./5 ft.
Special Attacks: Powerful charge, trample 2d8+9
Special Qualities: Darkvision 60 ft., escalating gallop, spell-like abilities, immunity to poison, charm, and compulsion, low-light vision, scent
Saves: Fort +10, Ref +7, Will +3
Abilities: Str 23, Dex 14, Con 21, Int 6, Wis 13, Cha 10
Skills: 9
Feats: 3
Environment: Temperate and warm plains
Organization: Solitary, pair, or family (3-6)
Challenge Rating:
6?
Treasure: None
Alignment: Always chaotic neutral
Advancement: 7–12 HD (Large); 13-18 HD (Huge)
Level Adjustment: –
Combat
Monoceroi would rather flee threats using their extraordinary galloping ability, they normally only attack to defend themselves or their family. They either charge, impaling foes with their horns like lances, or strike with their hooves. A monoceros will fight to the death rather than let itself or a family member be captured. The horn is a +3 magic weapon, though its power fades if removed from the unicorn.
Escalating Gallop (Ex): A monoceros can travel at incredible speed by taking consecutive Run actions - it can move 300 feet in the first round, 400 feet in the second round and 500 feet on the third and subsequent rounds it takes a Run action. The monoceros must gallop in a straight line.
Powerful Charge (Ex): A monoceros deals 2d10+15 points of damage when it makes a charge.
Spell-Like Abilities (Ex): A monoceros can use
detect poison at will as a free action. Three times a day, a monoceros can cure poison and disease with a touch of its horn - treat as a 5th-level spell that combines the effects of
remove disease and
neutralize poison (DC16, caster level 12th).
Trample (Ex): DC 19 Reflex for half damage. The save DC is Strength-based.