can't really tell from picture. Most resembles an acorn.Assuming it is not a mongrel, summer squash do not exhibit that shape. Acorns come in both vine and bush varieties. Summer squash are bush except for a few exotic varieties.

That is looking quite consistent with an acorn squash. Stop worrying! It looks like a summer squash cuz a summer squash is simply an immature squash. Most summer squash when left on the vine will eventually get large, seedy and develop a hard shell. The one pictured would probably be a reasonably tasty morsel if harvested now. But wait and your patience will be rewarded.