I mostly agree with elana but I don't think you should deprive a horse of water to get to cozy up to you. If you work a full time job you are depriving a horse of water for 8 hours at least. That's cruel. Grain you control and the comes to you to get. Water the horse in this heat regardless of how big a dink its being.
Also for what its worth, don't let your horses past dictate its future. It had a rough start that does not mean it get to be a dink for the rest of its life.