Did you plant seeds that you saved from last year? If so, you probably got an accidental cross. Squash cross pollinate like crazy. If you planted more than one variety, or even if your neighbor planted something, they could have cross polinated. Commercial growers have to separate squash types by a mile or so!
It is probably a hybrid zuchinni, and you can come up with some wild looking things. We have had all kinds of weird sports crop up the next year when we plant things too close together. Eat it before you save the seeds, might taste like yuck.
Cheers, Carolyn.