A dogs’s penis may suffer injury or damage during mating.
Swelling of the penis and/or the surrounding preputium often develops immediately after physical trauma. If a vessel is ruptured, blood may leak from the preputial orifice. Affected animals lick their penis, utter pain or show difficulties passing urine.
After mating, the penis remains lodged inside the vagina and the male dog may be unable to free himself. He may becomes frightened and make evasive movements, which may result in injury.
Treatment depends upon the nature of the injury. All bleeding will have to be stopped, and the penis has to be rested under all circumstances.
Owners should not attempt to separate "hanging" dogs by force. The situation is normally resolves itself in the regular passing of time. You may try to place a towel damped with cold water under the penis to fasten the loosening.