Why did they flee on foot, and not on their horses?
Radak: They feared to take the time to mount their horses 1 (Ralbag - due to their great panic).
Malbim: They thought that this will help them to flee. They thought that the enemy is already by them; they thought that the enemy will delay to despoil the tents and horses 2 and not chase them, since they left all the spoils.
Surely, it would have been wiser to spend a minute to mount a horse in order to flee faster, unless you will say that men run better than horses in the dark ( PF)
Why did they need to leave horses for this? Machaneh Aram had gold, silver and clothing (the Metzora'im took and hid some), but perhaps there was not enough to interest a giant enemy camp (most soldiers would get at most a pittance). They had so much food that prices plummeted in Shomron - but the enemy probably has enough food, and is not so fervent about taking more. Why would the enemy be so eager to take the horses? Presumably, each soldier already had a horse or chariot! Do not say that they wanted to get better horses. The best horses are from Egypt (refer to Devarim 17:16:3:1, 17:16:2:1 and the note there)! Perhaps they would want extra horses to carry the spoils. However, if the donkeys (which are normally used to carry loads) sufficed for Aram, they should suffice for the enemy! Perhaps the enemy would realize, if they left their horses and fled on foot, surely they are not loaded with valuables, so it is better to despoil the camp than to chase them. Could they make such a clever decision amidst panic?! I prefer to say like Radak and Ralbag, that amidst panic, they erred. (PF)