Woof. Beg. Roll over. Woof! Good puppy.
There's as much chance those words can be heard at a dog-training school as they can at a place like the Folsom Street Fair. In the midst of assless chaps, visible cock rings, and latex-bedecked people being beaten, there can also be seen people dressed as puppies. It is a lesser-known fetish but no less official than some others and is defined on Wikipedia as play “where at least one of the participants acts out canine mannerisms and behaviors.” The definition goes on to say that the play usually involves a trainer or master who might command the submissive to bark, kiss the dominant's foot, or eat like a dog.
But, “unlike other forms of animal role play, it is not uncommon for two or more pups to play together as equals,” as puppy play is often purely about the fun of it.