I think that if you're seeing butternut squash near this squash plant, they are on different plants.  How a 'mixed breed' plant occurs is if the plants were not carefully pollinated in a prior year that yielded the seeds with mixed up genetic material that you bought.  Sometimes we can tell the two apart as in a squash that had a pumpkin bottom with a top formed into the shape of a zucchini.  In your case, I can't tell.  Oftentimes these mixed up squash do not taste very good but you could try it.  I certainly wouldn't save the seeds from this one because it probably has cross-bred with the other butternut squash plants in your garden; and, likewise, I would find the seeds of the other butternut squash to be suspect as well.

I hope this information helps.