Hmm, I don't much care for a Colossal 36 HD version. The Yellow Belly isn't that big compared to a whale. It may be 100 feet long but its "extremely long and tapering" tail presumably takes up a lot of that so it oughtn't to be as massive as the largest sperm whales which have comparatively much bulkier bodies.
I prefer Gargantuan, like my original Working Draft.
Modifying that to use the Cachalot as the "base whale" produces:
Yellow Belly
Gargantuan Animal (Aquatic)
Hit Dice: 12d8+84 (138 hp)
Initiative: +1
Speed: Swim 40 ft.
Armor Class: 16 (-4 size, +1 Dex, +9 natural), touch 7, flat-footed 16
Base Attack/Grapple: +9/+33
Attack: Bite +17 melee (4d6+18 [?])
Full Attack: Bite +17 melee (4d6+18 [?])
Space/Reach: 20 ft./15 ft.
Special Attacks: Improved grab, swallow whole, vortex maw
Special Qualities: Amphibious?, blindsight 120 ft. [?], low-light vision
Saves: Fort +15, Ref +9, Will +6
Abilities: Str 35, Dex 13, Con 24, Int 2, Wis 14, Cha 6
Skills: Listen +15*, Spot +14*, Swim +20
Feats: 5 [Alertness, Diehard, Endurance, Improved Natural Attack (bite), Toughness]
Environment: Warm aquatic
Organization: Solitary
Challenge Rating: ??
Treasure: None
Alignment: Always neutral
Advancement: 13–18 HD (Gargantuan); 19–36 HD (Colossal)
Level Adjustment: —
Description.
Background.
A yellow belly is is about ?? feet long.
Combat
Tactics.
Improved Grab (Ex): To use this ability, a yellow belly must hit with its bite attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can immediately attempt to swallow the foe.
Swallow Whole (Ex): A yellow belly can try to swallow a grabbed opponent of a smaller size than itself by making a successful grapple check. The swallowed creature takes [XdY+12 or XdY+18?] points of crushing damage plus Z points of acid damage per round from the yellow belly’s gizzard. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal 25 [?] points of damage to the gizzard (AC 14). Once the creature exits, muscular action closes the hole; another swallowed opponent must cut its own way out.
A Gargantuan yellow belly’s interior can hold 2 Huge, 8 Large, 32 Medium, 128 Small, or 512 Tiny or smaller opponents.
Vortex Maw (Ex): A yellow belly can open its mouth so quickly that it creates a powerful suction. All creatures within a ??-foot cone must make Strength checks opposed by the yellow belly's Strength check (+24 for a standard yellow belly), with the same modifiers as a bull rush. If the yellow belly beats a creature's Strength check result, it pulls the victim 5 feet closer to its jaws. For each 5 points by which its check result is greater than a victim's check result, the yellow belly pulls the victim an additional 5 feet closer. The victim provokes attacks of opportunity if it is moved.
Skills: A yellow belly 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.
*A yellow belly has a +4 racial bonus on Spot and Listen checks. These bonuses are lost if its blindsight is negated.
I like that a lot better. I'm thinking we ought to increase the bite damage to, say, 6d6+18 so the average damage is close to a Colossal Cachalot 6d6+16 [that doesn't include any ability increases for the Cachalot's advanced HD, which could easily increase the Strength bonus by another 2].
The only problem that leaves is the reference to "maximum hit dice", so how about we give the Yellow Belly the maximum HD for a Cachalot Whale's Gargantuan advancement range of 13-18, i.e. 18 HD. I think that works better than using the Colossal HD maximum:
Yellow Belly
Gargantuan Animal (Aquatic)
Hit Dice: 18d8+126 (207 hp)
Initiative: +1
Speed: Swim 40 ft.
Armor Class: 16 (-4 size, +1 Dex, +9 natural), touch 7, flat-footed 16
Base Attack/Grapple: +13/+37
Attack: Bite +21 melee (6d6+18 [?])
Full Attack: Bite +21 melee (6d6+18 [?])
Space/Reach: 20 ft./15 ft.
Special Attacks: Improved grab, swallow whole, vortex maw
Special Qualities: Amphibious?, blindsight 120 ft. [?], low-light vision
Saves: Fort +18, Ref +12, Will +7
Abilities: Str 35, Dex 13, Con 24, Int 2, Wis 14, Cha 6
Skills: Listen +18*, Spot +17*, Swim +20
Feats: 7 [Alertness, Diehard, Endurance, Improved Natural Attack (bite), Toughness]
Environment: Warm aquatic
Organization: Solitary
Challenge Rating: ??
Treasure: None
Alignment: Always neutral
Advancement: 19–36 HD (Colossal)
Level Adjustment: —