Chelydra serpentina, twice. Both times I came up on them as they were in the left wheel track of my lane, which is where I usually ride. The first one was lifted and carried, hissing but manageable, to the shoulder and pointed toward the forest to which it was headed. The second one was notably larger and had quite a combative disposition. His neck extended around and back toward my hands to a degree that caused me to put him back down, quickly, and write off that particular rescue effort.