Any wranglers I've met would not have attempted to lasso an animal of that size running past at speed while their mount was standing still. The risk of injury to their mount themselves and the animal being captured is too great, compiled with the probable damage to their equipment. I know his thoughts were to use a loop of the lariat around the saddle horn for enough resistance to allow him to halt the running horse allowing his lariat to slip slowly through his hand, but there are a lot of problems with a maneuver such as that. Personal injury as he found out, or the lariat could have fouled on the horn, at best it would have snapped. At worst it could have injured the captured horse by choking it or possibly even breaking its neck, the saddle horn could have broken off, highly unlikely for a well made saddle, The mount could have had his right fore leg injured or even broken when the full force or the running steed jerked it off balance the cinch of the saddle could have broken causing rider and saddle to fall to the ground and be dragged by the horse. A western saddle has the cinch located further back than English saddles, this allows for better holding but also in an extreme circumstance if jerked hard enough can cause internal organ damage.
All in all there were so many things that could have gone wrong in an attempt such as in the video I don't even like to think about it