Basically, this is a saying used to describe something that a hell of a lot of people have.
For example, saying "everyone and their dog has a Ford Mustang" would be appropriate.

Or take the following conversation:

emo kid: "check out my ears, I just got them tapered, I'm so original"

person with an identity: "everyone and their dog has gauged ears in 2005"
by bryan18 August 16, 2005
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