Just as an observation, the difference between a winter squash and a pumpkin is.. well.. nada. We call some winter squash pumpkins because they're orange or it's tradition. Not because there is any culinary or botanical difference (there's neither).

Now, to answer your original question it could be a hybrid. were the seeds ones you saved or volunteers or were they newly purchased?

The shape looks similar to a buttercup some varieties but it's not something I recognize.