3e narwhal stats can be found
here.
I believe they were updated to 3.5 in the Tome of Horrors.
Those stats should prove useful for the alternate form.
Four Hit Dice for a Huge Animal?! Seems a bit low for a Narwhal.
Anyhow, getting back to the Sea Unicorn, I would just have the alternate forms provide different movement rates - high swim speed in "whale" form, high swim speed and low land speed in "hippocampus" form, high land speed, the amphibious trait and water walking in "unicorn" form.
I also don't like having a "whale" that breathes water instead of air. Do you mind describing it as a "fish"?
Whales have 120 ft. blindsight in 3E, so I would give the Sea Unicorn (even if we call it a fish) blindsight, although maybe cut it down a bit.
Statswise, I'm thinking something like this for a start:
Sea Unicorn
Large Magical Beast (Aquatic)
Hit Dice: 3d10+18 (34 hp)
Initiative: +3
Speed: 20 ft. (4 squares), swim 80 ft.
Armor Class: 16 (–1 size, +3 Dex, +4 natural), touch 12, flat-footed 13
Base Attack/Grapple: +3/+12
Attack: Horn +10 melee (1d6+8)
Full Attack: Horn +10 melee (1d6+8) and 2 hooves +3 melee (1d4+2)
Space/Reach: 10 ft./5 ft.
Special Attacks: Alternate form
Special Qualities: Blindsight 60 ft., darkvision 60 ft., immunity to poison, charm, and compulsion, low-light vision, scent, spell-like abilities, water walking, wild empathy
Saves: Fort +9, Ref +6, Will +6
Abilities: Str 20, Dex 17, Con 22, Int 10, Wis 21, Cha 24
Skills: Jump +28 [
+2 out of water], Listen +11, Move Silently +9, Spot +11, Survival +8*, Swim +13
Feats: Alertness, Skill Focus (Survival)
Environment: Any water
Organization: Solitary, pair, or balance (3–10)
Challenge Rating: 3
Treasure: None
Alignment: Always lawful neutral
Advancement: 4–6 HD (Large)
Level Adjustment: +4 (cohort)
Despite their name, sea unicorns are found in large freshwater lakes and rivers as well as the sea.
Alternate Form (Su): A sea unicorn can assume three forms, a hippocampus with a long unicorn's horn (The front half of a unicorn with a fish's tail) with the stats given above, a half-whale/half-fish creature with the same horn, or a unicorn of normal shape. All three forms have a blue-white coloration and blue eyes. A sea unicorn can only change form if it is submerged in water. It does not change form if it is slain.
In whale-fish form the sea unicorn's horn does 2d6 damage, its blindsight range increases to 120 ft and it loses
water walking.
Whale-Fish Form
Speed: swim 80 ft.
Attack: Horn +10 melee (2d6+10)
Full Attack: Horn +10 melee (2d6+10)
Special Attacks: Maybe give it powerful charge?
Special Qualities: Blindsight 120 ft., darkvision 60 ft., immunity to poison, charm, and compulsion, low-light vision, scent, spell-like abilities, wild empathy
In equine form the sea unicorn gains the Amphibious quality so it can breathe air but loses its blindsight.
Unicorn Form
Speed: 50 ft. (4 squares), swim 50 ft.
Special Qualities: Amphibious darkvision 60 ft., immunity to poison, charm, and compulsion, low-light vision, scent, spell-like abilities, water walking, wild empathy
Skills: Jump +16, Listen +11, Move Silently +9, Spot +11, Survival +8*, Swim +13