My volunteer "Pumkini" came up in my flower bed and I let it grow. The fruit is large and egg-shaped with a green skin (so far). It is immature. Can I eat it like a zucchini anyone know?
According to Okiedawn of Houzz/Gardenweb, you can eat young unripe pumpkins the same as summer squash. If it's more mature, you might be able to use it more like a winter squash (and I might add that it may ripen the rest of the way indoors, if you keep it in storage a while). I imagine your cross would be similar to that or zucchini. Crosses between C. maxima and C. pepo are supposed to be rare, though (so, if there's an alternative explanation, I would consider it, too).
I've eaten unripe winter squash before. I'd recommend cooking them (like frying them up like you would with a yellow summer squash), though. If they're bitter, I wouldn't eat them (bitterness tends to indicate higher levels of cucurbitacins, which are toxic).