“the donkey and her colt.” Much has been written about this, and some scholars have tried to make this into one animal, but the text clearly indicates two; a donkey and a colt. Piecing together the Gospel records indicates that Jesus sat on the colt, and it was so young that no one had ever ridden on it before (Mark 11:2). In the case of an animal that had never been ridden, it is wise to make sure that the animal will be as calm as possible, and that explains the second animal, the donkey. It seems the disciples, who lived in a culture in which it was common to ride donkeys, understood to bring the mother along with the colt, and Jesus sat on the colt, as Mark indicates.
“he sat on them.” Jesus sat on the garments, not on the two animals.