Important follow-up rules:
• If a person begins the voluntary prayer mounted and then dismounts, the prayer may be continued on the ground.
• Beginning on the ground and then mounting during the prayer does not have the same allowance.
• Impurity on the animal, saddle or stirrups does not prevent the mounted voluntary prayer according to the more correct view stated by the source.
• Performing prayer while walking is not valid.
These are specific rules; do not replace them with a vague statement that movement never matters.