The Plain Cow looks like an ordinary carton of milk with an image of a blue cow on the side. Plain Cow got it's name from the flavor of milk it was.
The Plain Cow is a wise and kindly saint or guru-like entity. He is being that only knows peace, love and compassion as well as the nutritional values of calcium and vitamin D.
When Billy was finally disowned by Jeff, the Plain Cow taught Billy about the ways of peace and compassion, and eventually told him how to stop the Spider Queen and regain Jeff's love. In the end, because of the Plain Cow, Billy was able to bore Arachnotaur, stop the Spider Queen and regain Jeff's love and even love him in return as a son.
After all the chaos has ended, Plain Cow praises Billy for his new-found wisdom and compassion, but when Billy questions the Plain Cow if it is a space minotaur like Arachnotaur, the Plain Cow calmly states that he was not actually there and that "milk cartons don't talk", and with that the Plain Cow mysteriously disappears into dust in the wind. Whether it was a figment of Billy's imagination, an embodiment of his conscience or something more is never revealed.