Well, they are the hardest to genetically breed, because you can
Click to reveal..
only breed the two sets of head at the very bottom with each other, or you get normal kids. However, you can get the heads at the bottom sometimes through your initial adoption, the adoption event, or a genetic mutation.

I hope I answered your question!
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I'm going to take the time right now to plug my fanfics. Aspen's story and Lilihia and the Tree.