You can heat a greenhouse full of cannabis like this.
Just get yourself a few containers of manure and run portable pipes through the center of the manure. They can be made air tight with a pressure valve and the methane can be filtered through charcoal and harvested for burning without stinking up the neighborhood. 
horses. you mean “get some horses”. donkeys will suffice but don’t crap as much as a horse. i recommend one of each. donkeys keep the coyotes away.
Mother Earth News magazine did a research project on this in 1973. It works but the pile needs to be very big. Too big to have inside a structure.
If they haven’t put it inside a structure then so much of the heat just blows away, ofcourse you’ll need a giant pile if you leave it outside, especially in winter.
If you put it all in a container, close it up, and make sure it’s well isolated like a fridge or freezer then it would be a lot more efficient and you wouldn’t need as much.
