Not much in the way of stats for the long-necked sea serpent, really. I agree to combine them; if we want a separate entry for the long-necked sea serpent, let's just write an underbar to note that there are some that don't have as much bite damage.
So, for our merhorse, shall we stick with Gargantuan and 14 HD? That feels slightly low for a Gargantuan critter, but it's not out of the question.
I think we might as well start at 14 HD Gargantuan, since we didn't "up Hit Dice" the Marine Saurian which had a similar problem.
We don't have much interesting with this beastie. I guess we could say the "respiratory filaments" justify giving it the Amphibious trait.
What do you think we should do statwise. It's supposed to be a massive seal, so we could up-size the official 3E seal from
Stormwrack and
Frostburn to Gargantuan as a foundation?
Seal
Medium Animal
Hit Dice: 2d8+4 (13 hp)
Initiative: +2
Speed: 20 ft. (4 squares), swim 40 ft.
Armor Class: 15 (+2 Dex, +3 natural), touch 12, flat-footed 13
Base Attack/Grapple: +1/+2
Attack: Bite +3 melee (1d4+1)
Full Attack: Bite +3 melee (1d4+1)
Space/Reach: 5 ft./5 ft.
Special Attacks: —
Special Qualities: Hold breath, low-light vision
Saves: Fort +4, Ref +6, Will +2
Abilities: Str 13, Dex 15, Con 14, Int 2, Wis 13, Cha 6
Skills: Listen +5, Spot +6, Swim +9
Feats: Alertness, Weapon Finesseᴮ
Environment: Cold aquatic
Organization: Solitary, pair, or herd (5–14)
Challenge Rating: 1/3
Treasure: None
Alignment: Always neutral
Advancement: 3–4 HD (Medium); 5–6 HD (Large)
Level Adjustment: —
Hold Breath (Ex): A seal can hold its breath for a number of rounds equal to 6 × its Constitution score before it risks drowning. For a typical seal, this is 84 rounds, or over 8 minutes.
Skills: A seal has a +8 racial bonus on any Swim check to perform a special action or avoid a hazard. It can always choose to take 10 on a Swim check, even if distracted or endangered. It can use the run action while swimming, provided it swims in a straight line.