I once noticed what looked like a very large dust goober on a few images as I was shooting an event. It was with a camera that had the same lens attached to it for at least several weeks. Even more curiously, it seemed to be moving around between shots.

Just before I pulled the lens off to blow off the sensor in the field (something I would normally never do), I noticed a medium sized spider spinning a web in the lens hood.

In the case of the spider, it was longer than it was wide and all squiggly. Your example looks kind of like one of Roger Cicala's little 'post-it note' squares stuck to the back element of a lens in this lensrentals.com blog entry.

My guess is that some kind of insect more square in shape flew past the front of your lens at just the right moment and was captured in your frame.

Maybe it was a dead leaf from the abundant vegetation where you were.

It could even have been a piece of litter blowing in the wind.

Or aliens.