Otters of a large range of species are basically water-oriented members of the weasel family along with wolverines, badgers, etc.
In North America, we have the North American River Otter. They are neither common nor rare. In the Northeat US, it has been my theory that they raise their babies on hatchery trout. Otters, like all weasels, have a high metabolic rate and are voracious.
They are not seen often, but they are around most stocked trout streams and rivers.
Seen one lately?