You apparently get 1 of 3 cupcakes when it's your pets birthday. So create a pet and get your free cupcake today. Mixed results on what the cupcakes are doing. I'll update when I get more information.
Yellow one The pink one So blue should
does str. does HP be def
The way to claim your cupcakes is view your pets look up . It's a non trade item, and some have reported 1 increase in the stat, while others have claimed 5. With the 2 that I have collected so far, they've done a +5.
ATM you can feed the cupcake to whatever pet you want to. It'll be in your inventory after you've received it.