Can anyone tell me what kind of squash this is?
To me, it looks a bit like a birdhouse gourd, albeit a different variegation.  Or perhaps a kabocha squash, which to me has a rather mild and mediocre taste and texture.

I guess you've ended up with some wacky gourd hybrid, resulting from open pollination of your desirable (F1?) hybrid. As is the nature of hybrids, the result could be caused by pollination either with itself, or something else in the area.  Either possibility could be responsible for the Frankenstein's monster you produced.  If you started with a zucchini, you've probably got some Cucurbita (e.g., vegetable marrow as @NBenatar suggests).  

Have a look at other links from Wikipedia's list of gourds and squashes, or one of a number of gourd identification charts such as this one that will help you narrow it down based on the nature of the inside of the fruit, leaves, etc.

On the bright side, you may have a squash on your hands that has never existed before!  On the other hand, it may not be worth much at the market. :)

"It's alive!!!"