A liitle fuzzy, but appears to be a squash. Squash cross readily within species so this would be a mutt of some type. Possibilties include acorn - zucchini cross. Probably came from you compost. If youl remember what you may thrown in there, you might be able to narrow it down a bit.

As always I agree with farmerdill anything he says.
It looks like a zucchini cross, to me.

Shape and ribbing look like a Delicata, but no striping. What does the flesh look / taste like?

Steve