Why not the Spyderco Police Necklace Knife, or whatever it's called. The miniature titanium handled folder. I would think that would be IDEAL!
One thing to consider, seriously, is how secure the blade is. I would want my dog to carry any folder that is only a liner lock, as once the blade gets jarred loose, it can come all the way open and hurt the dog. A good lock-back/front-lock design would be my first choice. Maybe a Spydie Dragonfly?
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iktomi