I enjoyed the first two ponies nice titties bouncing while the other two should either have arms tied behind the back or support the udders and clamp a weighty chain to bounce between the nipples.
Because it's a commercial video and the actresses just could not do it. Choice would have been to cancel shoot and reschedule (losing money), drop the two or allowing them to support their tits.
But these ponies deserve a real master, not a softy