Comment All in the packing (Score 2) 210
Donkey's can be frightened pretty easy yes, Horses if they're not trained, but War-Horses can handle extreme battle conditions pretty well.
Food vs. Batteries is a pretty even trade off. The robot is screwed if an EMP goes off, Horses are going to starve if there isn't any grass. A horse can beat 20miles in a day.
The only really good reason I can think of is packing. You could probably stick this on the back of a Humvee, which isn't really possible with a horse.