Been searching for this holy grail for years after an encounter with a client's youngish pitbull. I was always on the prowl, and when he left me alone with his cute dog for awhile, I proceeded to introduce the dog to my cock post-haste. He sniffed me, took a few licks, then, to my utter amazement just took my whole hardening cock in his maw and started greedily sucking it for all he was worth. I had never felt anything as good in my life as his throat rubbed my frenulum and the suction and tongue rubbing- I came in less than 20 seconds, which he eagerly swallowed.
He didn't even miss a beat and continued sucking, but I was worried the dude might come back at any moment, so I disengaged and thanked him, made a fuss over him and started playing tug-o-war with his rope toy, partly to explain why I was out of breath should the owner return.
I have since been sucked off by a chocolate lab and repeatedly by a friend's English mastiff. That mastiff was one talented dog in many ways.
Someday I'll get a puppy and "unwean" them hopefully.