Ermine (Mustela erminea), which is pronounced “er-men,” have three names. In North America, they are always called ermine or long-tailed weasels. In Europe and New Zealand, they are called stoats.
In North America, they tend to be found in the northern US and Canada, and most of them turn white in the winter.
Ermine coats are invariably of winter phase stoats from North America.
There is a related species called the long-tailed weasel(Mustela frenata), which is found from southern Canada to Bolivia. Some of the North American populations also turn white in winter. The main difference between the species is that the long-tailed weasel has a significantly longer tail in proportion to its body size, and they are normally quite a bit larger. However, there is a size overlap. The smallest long-tailed weasels are about the same size as the largest stoats. Long-tailed weasels are much larger than least weasels (Mustela nivalis), which are the smallest Carnivorans. Least weasels, like the stoat, are found in both Eurasia and North America, and they can even be found in parts of North Africa.
I have a long-tailed weasel in the freezer. It was killed in the late 90’s during squirrel season. It was chasing a rabbit when it was shot, and it was in the transitional phase between its winter and summer pelt.
Update: I have uploaded photos in a post of my long-tailed weasel that I mention here.
I’ve seen some long tails near my home in MD but they must be fairly rare as we have a huge vole population here and none of the local predators seem to be acting to control it to any extent.
Least weasels are fairly common in West Virginia, but the long-tails are fairly rare.
The ones I’ve seen here in n.-central MD are a lot bigger than the least weasel and are always near water of some kind–usually small streams. It can be difficult at times to tell the mustelids apart from a distance and in the past they have had mink escape from local fur farms, but I’m reasonably confident that what I’ve seen are long tails. A few years ago I saw what I am sure was a mink along the North Branch of the Patapsco River–definitely a different animal.