In the interest of brevity, this is what I had in mind. Either my Master or Mistress had decided that I have been naughty ( a rather common occurence) and I need to be taught a lesson. They call up a friend who brings his dog over (and I'd prefer a larger type dog) and make me service it.
Or - my lover catches me looking at doggy porn on the web and angrily pulls me out back and tells me if that's what I really want, then I should do his dog.
Or, fed up with the barking of the neighbor's dog, I go knock on his/her door to complain. Angry, he forces me out back to the kennel.
In my personal fantatsies, there is a chain link dog run that sometimes I'm forced to stand or sit up against and I'm chained to it and then the dog(s) are realeased. Sometimes they just are curious, but other times I envision my partner taking raw hamburger and smearing it all over me and pushing it up inside of me too.
After that, well, sometimes I'm put into the kennel for the dog's pleasure or sometimes I'm staked to the back lawn on my hands and knees.
Is that a good start for inspiration?