My brother-in-law just squeezed this out of a lump on his dog's back. I assume it must be a bot fly larva, although it looks different from the examples I've seen, maybe because it was squeezed out prematurely? There is one other example in the guide (that I know of) of a larva being removed from a dog:

Added 9/20/11: Some close-ups of this larva's sibling
here.