I recently came across this theory when search for some old Nicktoons to watch. It talks about Poof not being Cosmo’s, let’s look at the evidence clearly see if Poof is Cosmo’s or not.
In fairies, genetics shows through color. For example, Cosmo and Mama Cosma, both are green, albeit Cosmo is a darker green than Mama Cosma. We also see that Wanda and her sister Blonda have the same pink eye color. Poof isn’t green or pink though. He’s a purple-ish blue-ish color. The same color as none other than… Juandissimo. But we see Cosmo pregnant with Poof in the Fairy Baby special. So how isn’t he Cosmo’s?? Well, Timmy wished Poof into existence. So I propose that because Cosmo was infertile, the wish created a fairy baby using both Wanda and Juandissimo’s genes and then transferred the embryo to Cosmo.
Although it’s not said in the show I believe Poof is actually Timmy’s son because he did make the wish so he would be the father and plus Poof has similar eyes to Timmy.
Cartoon Universe 