WOW, that is one strange lookin' zucchini! Did just one plant produce that unique zuke or are other zucchini plants doing the same sort of thing? Here is just a thought, but are you having a higher than average amount of rainfall during this growing season? I don't know if that would cause this unusual growth or if it is a defect in the seed. BTW, which variety of zucchini produced this growth?
I would try to get someone to give you a more definitive answer. Perhaps, taking the zucchini to your local nursery and see if a horticulturist can explain this strange growth. Or you might want to try your state agricultural extension center. They may be able to shed some light on this interesting phenomena.
Please post again when you find out the answer. I would be very interested to know what has caused it.
Pretty sure this is caused by a disformed flower. We've had several like that over the years. Rest of the plant will be OK most of the time. Other misformed Zucchini are the work of squash mozaic viris. Pull & discard those plants as it will spread to others.
I used to see it fairly often when evaluating F2 seedlings from new crosses. It was a trait that was selected away from... everted ovlaries... a trait to avoid. Ugly, huh? If you save seeds, I'd suggest avoiding this one. Some types seem to be more succeptible to it than others.