We made Milky White. Ours was made to look like an old pull toy. So it was stuffed and on a brightly painted dolly. I saw another once made of papier mache with castors up to hooves. There was a blanket over the "handle" on th back to hide it and the eating mechanism was very clever. Under the blanket on the upstage side was a hole in the side of the cow. When he fed the cow, his wife would be on the upstage side of the cow pulling the objects into the cow that were pushed into the mouth opening. I've seen four productions of Into the Woods and everyone made their own Milky White. Each was different.

My highschool did a production, and I'm unaware of any rental companies. We got a LOT of laughs out of a two-person cow, however, with the front person making some great reaction faces...