You chose the animal companion stats for the horse which have 16 HP at level one when they are animal companions (ancestry + (6+CON)*level). The other options would have been level 0 riding pony with 16 HP or level 1 riding horse with 22 HP.
I think they should be riding horses. They are large draft animals, so they should have more hit points. This eliminates the possibility that a bad roll outright kills a horse when it overturns the cart or steps in the trap. Also, the riding horse stat block has an acrobatics modifier to use for the trap.
True, it is not a companion for anyone. Riding horse must be the correct option.