Spencer has had numerous cysts that have ruptured & have looked just like you describe. As Betsy said, antibiotics are usually prescribed. Also as Betsy said, the cyst can fill up again. Spencer currently has a cyst on his back that has ruptured, healed, filled & ruptured again 3 times. The vet suggested removing it, but at Spencer's age (12 1/2), I have opted to avoid surgery. Since it is just a cyst, it is not dangerous as long as there is no infection. It's definitely worth a trip to the vet though, just to be sure.