WARNING: This post sort of contains spoilers for some creatures and mechanics in the Handler's guide. If you wish to remain unspoilt as an Agent, read no further.
Delta Green is filled with various critters, some unnatural, and some completely mundane. However, all of them usually have what the Handler's Guide calls a "Defensive Property." This blogpost exists to expand upon the "Creating Your Own Unnatural Threats" chapter. I think it's a good resource, but it glosses over the finer details of DPs, plus I have my own bones to pick with a few of them.