If I had to guess, I'd say the most common guess(es) have been that Copper (the dog) is...

(Part)
• Pitbull
• Lab (any)
• German Shepherd
• Rhodesian Ridgeback

... to name a few. Well, apparently everyone was wrong. The results of Copper's DNA test show he is...

• 50% American Bulldog
• 50% German Spitz