How Do I Treat A Rash/hives On My Dog's Belly?

Benydryll (1miligram  for every pound the dog weighs) and olive oil
Rub the olive oil on their tummy. It wont hurt them if they lick it off.

There's a home remedy paste that you can try. It seems to work on most rashes. Mylanta and Destin (baby diaper rash med.), mix the two into a paste, and spread it on not too thick, not too thin, just an even coat all over the rash, and keep it covered for a couple of days. You should see a difference in about a day or two. Good luck.

Try a distilled vinegar water bath.