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So most of us think it's a person, and possibly even her grandfather. However it can't be her grandfather due to John being his father, and John being his own father, making it impossible for Jade to make John, which would make her grandfather her grandson. (Confusing, I know)

Maybe however, it's Bilious Slick.
I know Kanaya said Jade had to make Bilious Slick but maybe she wasn't able to directly and created his mother. Then, through the use of Dave's weird time s**t, he was able to grow up and become who he is and get stuck in the past or future. (Idk, weird time s**t) This also may have made Kanaya Bilious Slick's mother/grandmother also.

Chances are slim, but it's the only thing I can think of due to Jade being the mother of many frogs.