There are different things it could be. You can do a search online for animal droppings to maybe confirm what you are seeing. It could be a possum, skunk, or even a raccoon, but I'd bet on a rat. If you have one rat, you have several.
If I can, I really like to know what I am going after, but sometimes that is not possible. I don't like poisons because of the risk to other things but if you are set up where you can do that safely, you can try rat poison. I also don't like snap type traps but if you can set tha tup where your dogs, cat, or chickens can't get to it, that is a possiblility. I use those for mice.
Yoou can also get a live trap, Havahart or some such. Your hardware or feed store should have them. Get one the right size for ratas, maybe 6" to 8" wide and high and maybe 16" to 20" long. I really don't like the 2-door type. They are more expensive and I think that second door just gives them another escape route when it springs. Peanut butter can be a great bait. I just put some in a paper napkin, wad it up, and throw it in the back of the trap.
You don't have to but I built something like this to go over the trap so the only way in is through the front door.