Although BS promised it some time ago, there wasn't a dead mule in the prologue! RJ said that he wanted to include one in the prologue of AMoL, because it's typical for southern USA literature. Didn't notice it myself, when I read the prologue, but now that I'm aware of it, it's somehow a pity that RJ's wish wasn't fulfilled, because BS apparently forgot about it.

On the other hand, BS has yet two more prologues to include a dead mule..

However, it's insofar interesting as RJ apparently wrote most of the prologue. And RJ stated that there will be a dead mule in it, which was confirmed by BS. So, normally I don't really see a reason for BS to exclude the dead mule then, if RJ wanted it in it. Maybe the split of AMoL forced BS for some reason to exclude certain parts of the original prologue and put them at a later point into the books. On the other hand, it shouldn't be so difficult to include a dead mule in the prologue we got, e.g. Tylee could notice one at the roadside in her PoV. Anyway, we will rafo if there will appear a dead mule yet in the books!