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 generally like pets in games, so I could get behind having a “guardian pet.” Would like some over sized cat options as well. In Warframe I bring my Crit Kitty with me everywhere. There are few things in games more satisfying than seeing your pet come flying out of nowhere to rip an enemy to pieces.