I am a dog trainer... I study dog psychology.
I almost guarantee this was a pack of feral dogs.
If dogs are allowed to roam and have little training- they easily teach themselves to kill things.
A pack of dogs could most definitely bring down a small cow. A pack of feral dogs could even kill a large breed of horse.
This sounds horrible... but it is true. In rural suburban and rural areas, criminals often leave aggressive, tenacious dogs outside to protect their illegal paraphernalia.
These dogs are not tied up, otherwise their "protective" qualities would be of little use.
For all we know, this killing problem might be a whole lot bigger than a few dogs setting out to "have fun".
It is a sad thing... but notify your community and call the authorities. And if I were you, I'd be spending a good several nights out at my coop. Get something from a hunting supply store to cover up your scent. Watch, and wait. If it is indeed a pack of dogs, you can identify the dogs and call the authorities and request they allow the dogs to be shot on site. Or, find the owners and sue them up the butt for the livestock expenses they owe to the town.