GRAPHIC photo added post 20. Poor gal..RIP.

[Mattemma]

If it is not raining put some flour or something around the coop to see if you get prints.That poor hen!

 

[hinkjc]

I would guess rats or weasel or maybe a young coon. Something with a small mouth.

 

[Miss Lydia]

That is the saddest thing I've ever seen. The pain she must have been in and to have lived. I hope you catch what ever it is.

 

[aka Rachel]

Thats just horrible, must have been awful for you. I sure hope you catch the culprit, my uneducated guess (just from reading the net) is racoon.