The worst part is how much your horse likes to "dance" around back there. The EXT will help you, but your horse may be more in control then you'd think.
I hauled a tandem stock trailer (approximately 21 feet) with some one year old bulls in it. I think that there were 6 of them if memory serves me right and the lightest one came in around 900 pounds and the heaviest was in around 1300. I towed this with my 82 Chevy 1/2 ton that I put 3/4 ton axles and springs in. What a white knuckled ride that was! Those bulls would shift to the side and the whole truck wanted to go with them.
Hauling livestock can be scary if they like to dance.
This is why I bought my Chevy dually. Takes a LOT to make that sucker move.