This may not be helpful to you, but there are 2 different Gamestop cards, $10 & $20. The $10 card gives the Crab pet and the $20 card gives the walking octopus. Hopefully there will be new pets with the cards after 3 months (the amount of playing time those 2 cards combined would get you).
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This is what it says on the Pirate 101 prepaid card page: Each prepaid game card comes with a Free Pet. The pets change each month and the pet you receive depends on when you REDEEM the card, not when you PURCHASE the card.
I'll see about getting that changed ASAP to say how things are currently working. On the upside, I can verify that it has both been discussed here and requested by players! On the downside, the wording on the website is incorrect for the time being, and I don't have any additional information regarding actual implementation.