Converting "Real World" Animals and Vermin
Shade
Monster Junkie
Grouper, Giant
CLIMATE/TERRAIN: Tropical salt water
FREQUENCY: Uncommon
ORGANIZATION: Solitary
DIET: Carnivore
INTELLIGENCE: Animal (1)
TREASURE: Nil (R,X)
ALIGNMENT: Neutral
NO. APPEARING: 1
ARMOR CLASS: 5
MOVEMENT: Sw 12
HIT DICE: 6
THAC0: 15
NO. OF ATTACKS: 1
DAMAGE/ATTACKS: 2d6
SPECIAL ATTACKS: Swallows whole
SPECIAL DEFENSES: Nil
MAGIC RESISTANCE: Nil
SIZE: H (16+’ long)
MORALE: Elite (13)
XP VALUE: 1,400
The giant grouper is a lurking predator that can be found near coral reefs and the wrecks of sunken ships. The largest specimens have been known to reach a length of 20 feet and weigh 1,000 pounds. Its huge jaws act as a giant suction pump, drawing smaller fish into its gaping maw. This giant fish sometimes stalks solitary divers like a great cat.
On a roll of 20, the giant grouper swallows a creature of man-size or less; otherwise, the creature is merely bitten. When a grouper swallows, it creates a great suction that sweeps the intended prey and all tiny creatures (size T) within 10 feet into its maw; none of the swallowed creatures are damaged in the round they are swallowed. A person swallowed with a dagger or similar small, sharp weapon in hand can cut his way out by inflicting damage equal to 25% of the grouper’s original hit points. The grouper’s digestive fluids inflict 1d4 points of damage per round, starting the round after the creature is swallowed. An enterprising character who has been swallowed might try to leave the grouper when it opens its mouth to swallow other prey, as well; this requires a successful Strength check.
Originally appeared in Dragon Magazine #116 (1986). This is the Monstrous Compendium Annual Volume Four (1998) version.
CLIMATE/TERRAIN: Tropical salt water
FREQUENCY: Uncommon
ORGANIZATION: Solitary
DIET: Carnivore
INTELLIGENCE: Animal (1)
TREASURE: Nil (R,X)
ALIGNMENT: Neutral
NO. APPEARING: 1
ARMOR CLASS: 5
MOVEMENT: Sw 12
HIT DICE: 6
THAC0: 15
NO. OF ATTACKS: 1
DAMAGE/ATTACKS: 2d6
SPECIAL ATTACKS: Swallows whole
SPECIAL DEFENSES: Nil
MAGIC RESISTANCE: Nil
SIZE: H (16+’ long)
MORALE: Elite (13)
XP VALUE: 1,400
The giant grouper is a lurking predator that can be found near coral reefs and the wrecks of sunken ships. The largest specimens have been known to reach a length of 20 feet and weigh 1,000 pounds. Its huge jaws act as a giant suction pump, drawing smaller fish into its gaping maw. This giant fish sometimes stalks solitary divers like a great cat.
On a roll of 20, the giant grouper swallows a creature of man-size or less; otherwise, the creature is merely bitten. When a grouper swallows, it creates a great suction that sweeps the intended prey and all tiny creatures (size T) within 10 feet into its maw; none of the swallowed creatures are damaged in the round they are swallowed. A person swallowed with a dagger or similar small, sharp weapon in hand can cut his way out by inflicting damage equal to 25% of the grouper’s original hit points. The grouper’s digestive fluids inflict 1d4 points of damage per round, starting the round after the creature is swallowed. An enterprising character who has been swallowed might try to leave the grouper when it opens its mouth to swallow other prey, as well; this requires a successful Strength check.
Originally appeared in Dragon Magazine #116 (1986). This is the Monstrous Compendium Annual Volume Four (1998) version.
Shade
Monster Junkie
It looks like another opportunity to use the vortex maw ability. 
Vortex Maw (Ex): A mastodonsaurus can open its mouth so quickly that it creates a powerful suction. All creatures within a 15-foot cone must make Strength checks opposed by the mastodonsaurus's Strength check (+8 for a standard mastodonsaurus), with the same modifiers as a bull rush. If the mastodonsaurus 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 mastodonsaurus pulls the victim an additional 5 feet closer. The victim provokes attacks of opportunity if it is moved.
This time, we might consider a Swim check opposed by the fish's vortex maw result to attempt to escape while others are sucked in.
Vortex Maw (Ex): A mastodonsaurus can open its mouth so quickly that it creates a powerful suction. All creatures within a 15-foot cone must make Strength checks opposed by the mastodonsaurus's Strength check (+8 for a standard mastodonsaurus), with the same modifiers as a bull rush. If the mastodonsaurus 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 mastodonsaurus pulls the victim an additional 5 feet closer. The victim provokes attacks of opportunity if it is moved.
This time, we might consider a Swim check opposed by the fish's vortex maw result to attempt to escape while others are sucked in.
Shade
Monster Junkie
I was thinking the Swim check to escape once already swallowed, but it could probably be expanded to the initial attack.
Sure, let's bump the HD a bit. An orca is Huge, with 9 HD. These start at about the same size, so use the same HD?
Orca ability scores: Str 27, Dex 15, Con 21, Int 2, Wis 14, Cha 6
Spot-checking a few fish, they've got Int 1, Wis 12, Cha 2.
So maybe Str 27, Dex 15, Con 21, Int 1, Wis 12, Cha 2? Possibly drop Str by 2?
Sure, let's bump the HD a bit. An orca is Huge, with 9 HD. These start at about the same size, so use the same HD?
Orca ability scores: Str 27, Dex 15, Con 21, Int 2, Wis 14, Cha 6
Spot-checking a few fish, they've got Int 1, Wis 12, Cha 2.
So maybe Str 27, Dex 15, Con 21, Int 1, Wis 12, Cha 2? Possibly drop Str by 2?
Cleon
Legend
That Dexterity seems rather high. They're not very fast or agile fish and rely on their suction-attack to catch prey.
Methinks Dex 13 or even lower.
