Hair of the Dog

We came home Friday evening to find that Sprocket had a large, red, swollen bump on his hind leg.  Fang marks are obvious, as you can see in the picture below, but the perpetrator is still a mystery to us. punctures are spaced just a half centimeter apart, so it was small - whatever it was.  Possibly a small snake or a tarantula?

Any one have any clues as to what this bite is from?

Thanks,
Sara