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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I see your point and it does make a strong point but the running theme of who the ghosts choose to be gardians is that they are sentient and are capable of complex thought and communication, although I would live to see bungie add pet companions into the game, it would add another interest dynamic and further character development, almost giving you more investment into the game