Note: this is not Holly, the Company Dog.
These are thanks to the appropriately-named, very focused, does-what-it-says-on-the-tin blog Food on My Dog. I love that there is someone who had the idea (and the dog) -- and then followed through!
Lettuce. If you go to Food on my Dog, there is a video of the lettuce being put on the dog's head (just below the maple syrup).
At this point, You will want to see this question to the blogger, and his answer: