Every environmental enemy (meaning, they don't react to Mario -- only environmental stimuli) I create insists on going left unless I contrive a scenario that forces them to go to the right. Even ant troopers, which can walk around any connected surface, go left when placed on top of a surface and still left when placed on the bottom of it, such that instead of making an interesting obstacle in which two ants are consistently opposite each other on a platform, they simply ram into each other and change direction on the side of the platform.
I've been able to contrive a scenario with the ant troopers to force them to walk to the right by placing an upside down pipe with a wall to the left of it. Once disgorged from the pipe, the ants will walk down the wall and, once they get to the floor, they will go right. But that's a lot of setup for such a simple ask.
I haven't looked at all of the enemies, but I know Banzai Bill has a directional arrow on his sprite that lets you change the direction he goes. This implies that enemies without the arrow go to the left and there's nothing I can do about it.
Thus, my question: is my hunch that enemies with no direction arrows on their sprites will go the direction they want to go, or is there a way in Course Maker to swap out their starting direction?