Weasel, ermine, ferret, mink, badger, fisher, ermine, wolverine, skunk, etc.
Mustelidae and Mephitidae spp
This group contains diverse predators including those listed above. Those species interested in preying on your chickens generally are small to medium, long-bodied, short-legged omnivorous mammals.
Some are tiny; for instance, some weasels aren't much bigger than a mouse, but they can kill rabbits and chickens many times their size. Some members of this group are very large; for instance, wolverines have weighed up to 70 pounds.
Various species are found throughout the world
Some in this group are not predators of chickens (otters, for example), and some are more often predators of young chicks or eggs (such as the skunk).
However, most types will eat chickens if given the opportunity, and when they do, their hunting behavior is similar enough to be classified together here.
These predators can often be very effective hunters. The lithe body shape of many types means they can squeeze through holes that mice or other small rodents can barely fit through. In fact, most of this group usually prefer to prey on voles, mice and shrews, so this body shape is quite efficient. They will also be attracted to unsecured animal feed, ungathered eggs and the like. The truth is that chickens are not the preferred prey of many members of this group, and it is relatively rare for most even to prey on small wild birds, much less chickens.
That said, if you give them the opportunity to attack your birds, they will certainly take it. Larger members like fishers, wolverines and badgers are an exception: they can be vicious determined hunters of your chickens if they are in your area. But since the primary diet of many small weasels consists of mice, a weasel who has gotten into your coop may initially have done so initially because it was hunting mice, or even because it found your unsecured chicken feed, hidden eggs or something else in your area that attracted it. When they eat most of the rodents and preferred prey in your area, they may also begin targeting eggs, chicks and adult birds.
Members of this taxon are typically excellent climbers and diggers, and larger types can be quite strong, forcing their way through the edges of wire mesh that aren't secured. Don't be surprised if the animal seems very large for the entrance you find! They are designed to fit into small spaces.
If these are the predators of your chickens, they will probably kill multiple birds in the way foxes will. They kill with lots of small bites to the base of the skull. The larger predators in this group may carry off a carcass, but since they tend to gain access through smaller openings, it is usually not possible to remove a carcass from the scene of the crime. Instead, the bodies may be strewn about where they were killed, or they may be piled. Which particular species did that damage can probably best be determined by which are common in your area. Predation by skunks will, of course, leave a lingering smell.
Protecting Your Flock
First, don't attract these predators to your yard. For instance, don't leave out cat food, and make sure they can't get to wild bird feeders or any seed beneath the feeders. If you have mice, be sure to take care of the problem before weasels or other predators of these rodents are attracted. In dry areas or during dry periods, they may be attracted to sources of water in your yard, too. Don't leave anything out for them!
To keep your flock safe, make sure any wire mesh on your coop is securely attached, because these predators can force themselves through areas that are not firmly attached. They can climb fences, and dig beneath enclosures.
Don't use chicken wire for your coop or run: chicken wire is not a barrier to predators. Most predators can tear through it like tissue paper, and some in this group can simply squeeze through the holes. (Chicken wire should only be used to keep chickens in, not to keep predators out.) Instead, use hardware cloth, and be sure to provide a small mesh for best protection (1/2" or 1/4").