In which movement does the horse go straight and sideways with its outside leg crossing in front?

The movement where the horse goes straight and sideways with its outside leg crossing in front is referred to as the side pass. In a side pass, the horse moves laterally while maintaining a straight line, allowing the horse's outside leg to cross over the inside leg in a coordinated manner. This lateral movement is often used in various equestrian disciplines to demonstrate flexibility, control, and communication between the horse and rider.

In contrast, the other movements mentioned serve different purposes. The half pass involves a diagonal movement that combines both forward and sideways motion, where the horse moves at an angle and the inside hind leg crosses in front of the outside hind leg. The rein back is a backward movement where the horse steps back in a straight line, while the rollback is a quick pivot or turn performed after a forward movement, typically used to change direction rapidly.

Understanding the specific mechanics of these movements is essential for instructing riders effectively and assisting in their horsemanship development.