In response to Don Pepito's question , yes invariably as a logistics unit ..all terrain, but slow moving.
In ancient/medieval scenarios I usually use pack mule trains (using Dark Sheers horse as the unit) to transport gold (Using the "treasure chest " unit,auto-produced by "tax collector" buildings) to newly conquested cities to speed city production. The AI obviously doesn't do this and will disband any mule train successfully attacked. Its time consuming but adds reality.
In a western US Cavalry scenario I use Mules to transport Gattling guns (this is historically accurate) or light field pieces., or "evacuate" wounded back to Forts (again historically accurate)
In my WW2 War in the Pacific scenario both Japanese and Chindits use mules in the Jungle or Mountains to transport light field pieces ( again the AI wont do this , unfortunately)
The great thing about CIV3 is that even within the limitations of the unit editor, if you are "creative" you can make up loads of things . As a stupid example in my ancient scenarios I flag elephants as "nuclear weapons" and then only have one type of sea transport capable (usually very expensive,slow and vulnerable) of "transporting nuclear weapons".
I dont know if that was helpful , but what I am trying to say is that there are loads of ways you could use pack mules as a unique logistical unit in almost any era