I use the Gamo Varmint hunter it comes with lasers and light and drops coons, rats and anything else under 100 lbs. Ive heard of peaple shooting deer with it before. I has the power of a 22. You can get it at any large outdoors store or online.
Sounds like a coyote. Use traps if they are legal in your state. Or get a bullet into them because now they know the chickens are there and will come back for more.
Get a big rooster. Our roosters keep the hawks away from the coop. We have many hawks on the ranch and will usually just watch the hens from a distance.
Get a coyote call find a good spot upwind. and call them in at night. I've killed lots of them. Calls can be cheep or expensive. Fox pro is the best. but other calls work good at short distance. Rabbit in distress is my favorite. Look into a .22 mag or a .218 with a good scope for long distance...