Most double horse trailers weigh around a ton or over, add the weight of one decent nag, and you're getting close to maximum towing weight, which in itself isn't a problem as horse trailers are usually very stable (ho ho) and tow well.

FWD won't be much use if you venture off hard concrete whilst towing.

Another thing is that horse trailers being mostly tandem axled give the towing vehicle a hard time, my purpose built fibre glass bodied Dutch model had fully independent and very soft supension but that's not usual, a sturdy long lived Ifor is likely to not do your car much good over time.

There's good reasons behind most horse trailer users using robust 4WD's, being able to take the punishment dished out by the towed trailer is just one of them.