Help identifying squash
My best guess based on your photo is that it's kabocha (aka Japanese pumpkin)



(image source).

But squashes also hybridize pretty easily, so far as I know, so it could be a random hybrid rather than an established variety. Possibly one derived from kabocha or acorn squash. 

If you grew this from seed saved from last year's garden squash, it could be that your squashes were hybrids that can't be grown from their own seeds.

Now I noticed you said the flesh is green. That makes kabocha less likely. Maybe a hybrid of kabocha and zucchini, that happened to get the skin and shape of the kabocha and the flesh of the zucchini?

Edit

I've researched further and learned that kabocha is a variety of the cucurbita maxima while zucchini is a variety of cucurbita pepo, two different species, and therefore these two could not hybridize. 

However, C. pepo does include both summer and winter squash varieties, so it would be possible to get a hybrid of the two if both parents were varieties of C. pepo. C. pepo includes acorn squash and (some) pumpkins among winter squash varieties, and pattypans, zucchini, crooknecks and many other summer squash varieties.

C. maxima includes kabocha, hubbard squash, and many others. I'm not familiar enough with other maxima varieties to say if there are varieties that are likely parents of your example.