A freight dog is a pilot that flies strictly freight and usually for one of the major freight haulers like Federal Express or UPS. Hauling freight can be a tough living since you work during the later hours of the night.
Freight (or packages) are processed through a central network at night. They are sorted and then go back out to their destinations after the sort is completed. Sometimes a pilot will bring in a load of freight, wait for it to be sorted and then take out the sorted packages to where he started. Other times a pilot will start or finish at the sort base. It is pretty hard to avoid flying the back side of the clock, unless you are very senior at those companies.
A lot of work gets done at night and the men and women that fly those hours are generally always trying to make sure their sleep pattern is standardized. I found my time of flying freight to be a challenge. The first part of the week goes ok, and by the end of the week you are starting to feel pretty wore out. The real challenge is trying to blend your home activities with your day sleep schedule. It is not for everyone, but if you are a night owl it could be the perfect calling for you.