If you are buying the Canon 500mm or 600mm f4L IS II, you can easily use them on a monopod. You can also, with some practice handhold them. If you are considering the version I of those lenses, it becomes a bit more challenging, both due to weight (primarily the 600mm) and because the IS system is much less efficient (2 f-stops instead of the 4 f-stops you get on the version II).
For wildlife and birds I handhold most of the time, unless I am sitting still, waiting for something. I also handhold with the 1.4xIII extender, but that requires practice. When I attach the 2xIII I need the tripod with gimbal head, both because of the 1200mm focal length and because I then only have one center AF point available (1DX and 5DIII).