There are millions of mineral and rock formations around the world with 90 degree angles. A tree/flower stem/etc and the ground form a 90 degree angle. A branch coming straight out of a tree trunk (common in pines, or some other evergreen) makes a right angle. A vine or spanish moss dangling from a branch makes a right angle. Two logs or branches that fall on each other in the forest serendipitously form a right angle. An arm, leg, etc sticking out of an animal with arms forms a right angle when extended. Water running down from two different points on a hill toward a central point could easily join as a right angle.

I'm pretty sure right angles existed before this log.