Shar-Peis/Large bump on back leg with pus
My shar pei has a big bump full off clear fluid that leeks nonstop, it's on the lower back leg.
could this be cancer?
This is more than likely a condition called "mucinosis". Mucin is the substance that fills the wrinkles in the shar-pei and makes them wrinkly. Some shar-pei "over-produce" this, and it causes mucinosis. While it is not anything to worry about health wise, it can be messy to deal with. Many times, it is associated with hypo-thyroidism, so having the thyroid checked out (blood test) would be the first thing I"d do. Also, with shar-pei, some do show a "low/normal" range, but still need to be put on thyroid meds (not expensive) for the rest of their lives, and the mucinosis will usually disappear. If the thyroid is fine, then it can at least be kept under control by keeping the area clean and dry. Betadine washes tend to help do this.
Other than this, there really isn't much you can do, except keep the area clean and dry, and don't worry about it. It's not a serious problem for the dog.