Otters are common throughout a great portion of the US. In some states they are protected, in others they can be trapped as furbears (in season, with a license). Check YOUR state laws regarding animals in your area and exemptions for damage control.
I'm sure an otter would be glad to make a meal of a chicken, but wouldn't expect it to be a common experience unless the coop is very close to a major waterway. They, like coon, are tenacious beasts with an amazing amount of strenth for their size. As stated above, they are large relative to mink or the diminutive weasel. If you are in an areas with mink farms, they occasionally have escapes and those mink will be signficantly larger than the wild ones.