In California years ago, when drop down windows first came out, a trailer was traveling along a two lane winding road and the horses heads were hanging out. You guessed it, the horses heads were over the yellow line going around a curve and a big truck was coming the other way. One of the horses was literally decapated and the other horse was also killed.
I have drop downs in my trailer and window grates, the windows are dropped down, the grates stay up. Plenty of air, the horses can see out, but no chance of them getting their heads out. I hate seeing that going down the road