I wouldn't buy a one horse straight load. But I agree, if you MUST, get a double axle.
I drive a red chevy S10 most of the time. I can haul my little two horse straight load comfortabley. However, I have had horses panic inside of it, pull back, rear, general stupid baby horse things while my 20yr old paint was giving them the "Stfu I'm trying to eat" from the slot beside.
Not for the inexperience horse. I couldn't even imagine loading my barrel mare into one at this point in time, she barely survives our stock trailer rides. (We're working on it. She gets haule back and forth to walmart nowadays because she's such a freak.)
However, I REALLY like the small stock trailers. Almost every horse will jump right in those.