Dave Arneson’s Blackmoor supplement is where the Giant Otter first appears in D&D.
I have never used this creature in a game, but I see that it could be a great encounter for any wilderness adventure.
Different varieties of otter live in many climes, so anywhere there is a river, lake or ocean, you could insert one into an adventure.
They are a carnivorous mammal and will eat just about anything that they can catch in their environment.Except when feeding, they are not typically aggressive. They use high-pitched vocalizations to communicate.
The “real life” Giant Otter is a species that lives in South America. You could use these stats for a more realistic Giant Otter in your campaign or as a basis for creating a truly huge Giant Otter:
Males are between 1.5 and 1.8 meters (4.9–5.9 feet) in length and females between 1.5 and 1.7 m (4.9–5.6 ft). The animal’s well-muscled tail can account for as much as 69 centimeters (27 in) of total body length. Early reports of skins and living animals suggested exceptionally large males of up to 2.4 m (7.9 ft); intensive hunting likely reduced the occurrence of such massive specimens. Weights are between 32 and 45.3 kilograms (70–100 pounds) for males and 22 and 26 kg (48–57 lbs) for females.
The otter is perfectly suited for an aquatic life. Long and sleek, it has short legs, webbed feet, and a long tapered tail.
The giant otter has the shortest fur of all otter species; it is typically chocolate brown but may be reddish or fawn, and appears nearly black when wet. The fur is extremely dense, so much so that water cannot penetrate to the skin. (This is why the Giant Otter’s velvety fur is so prized!)
They live about 8 years in the wild.
When hunting these creatures, I suggest you take along siege engines and make sure you acquire at least a +2 weapon vs. Giant Otters.
Personally, I think they would make a better Druid companion creature than a fur coat.