If Devotion, Bravery, Sacrifice, and Death are the requirements for being chosen by the light. I'm sure some pet dogs out there meet these requirements for sure. If your dog is devoted to it's master, brave enough to bark at any stranger that looks like it'll harm you, sacrifices itself to protect you at it's demise. In the Destiny world, You're dog may be a Guardian, and won't that be funny.
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Sparing a thought for Luna. <3 You kinda don't have the criteria right though as many warlords were risen light Bearers yet weren't Guardians. Still, i like where your head is at but I'm fairly certain the main criteria is "has thumbs" closely followed by "can shoot".