Okay, I've filled in the few gaps in my proposed stats for the "realistic animal" version of the Potamotherium and came up with the following.
I upped their speed because otters are pursuit hunters able to catch fairly fast fish by outswimming them, and decided to give them Spring Attack as a bonus feat because I remembered reading something about giant otters driving away or even killing Caymans by darting in-and-out to bite them before they can react, like a mongoose fighting a snake.
I also toyed with the idea of giving it a Special Attack like the Attach of the SRD's Weasel, but decided against it.
What do you think?
Potamotherium
Small Animal
Hit Dice: 2d8+2 (11 hp)
Initiative: +3
Speed: 30 ft. (6 squares), swim 40 ft.
Armor Class: 15 (+1 size, +3 Dex, +1 natural), touch 14, flat-footed 12
Base Attack/Grapple: +2/–1
Attack: Bite +6 melee (1d6+1)
Full Attack: Bite +6 melee (1d6+1)
Space/Reach: 5 ft./ 5 ft.
Special Attacks: —
Special Qualities: Agile swimmer, hold breath, low-light vision
Saves: Fort +4, Ref +6, Will +1
Abilities: Str 12, Dex 17, Con 12, Int 2, Wis 12, Cha 7
Skills: Balance +5, Climb +5, Escape Artist +6, Listen +5, Spot +5, Swim +11
Feats: Alertness, Spring Attack (B), Weapon Finesse (B)
Environment: Temperate aquatic
Organization: Solitary, pair or family (3-12)
Challenge Rating: 1
Treasure: None
Alignment: Always neutral
Advancement: 3-4 HD (Medium)
Level Adjustment: —
These slim fish-eating mammals may be a primitive ancestor of the modern otter, which they strongly resemble in shape and habits.
A typical Potamotherium is up to 5 feet long, including its tail, and weighs around 30 pounds.
Combat
These animals usually avoid combat, but when forced to fight they will circle around their opponent and use their Spring Attack feat to dart in, bite, and escape to a safe distance before their foe can respond.
Agile Swimmer (Ex): Unlike most creatures with a Swim speed, a Potamotherium does not have to move in a straight line when it makes a Run action while swimming. It can turn by 90 degrees for every 5 feet it travels.
Hold Breath (Ex): A Potamotherium can hold its breath for a number of rounds equal to 4 times its Constitution score before it risks drowning. For a typical Potamotherium, this is 48 rounds, or 5 minutes.
Skills
A Potamotherium has a +2 racial bonus on Balance, Climb and Escape Artist checks.
Potamotheriums have a +8 racial bonus on Swim checks and can always choose to take 10 on Climb checks, even if rushed or threatened. A Potamotherium can use either its Strength or its Dexterity bonus for Climb or Swim checks and can take the Run action while swimming (see Agile Swimmer).