An irregular discharge from a dog penis may indicate something completely normal or not. It really depends on what is being discharged. A yellowish discharge coming from the bit of flabby skin surrounding the dog's penis-called the prepuce-is completely normal.
Bloody Discharge
If there is a bloody discharge, this is more serious. A bleeding dog penis may indicate an inflammation of the prostate gland. Left unchecked, this can lead to further problems including renal disease or failure. At that late stage, dog kidney dialysis will be one of your few options. Blood can also point to irritation of a dog's overactive bladder or urethra.
Basically, a yellowish secretion is normal, blood is not. If you notice a bloody discharge coming from your dog's penis, it is best to take him into the vet for a checkup. The discharge from the prepuce can be treated at home-if excessive-with a feminine douche.