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Spencer smith plastics, do a metal mule train/pack animals in their Jacklex range, metals, but they are our scale and sit well with plastics, a sort of stop gap while we wait, there are also some about in Matchbox`s Anzac set, Airfix`s German mountain troops, and Airfix`s jungle house, if you can find it, but I agree a specific set would have a lot of uses over a very long period of history.
Just make available the mules+oxen sprues from the Crusader transport sets: you don't have to be a sculptor of Michelangelo level to make up a load from blobs of putty!
I still use my old Atlantic Gold Rush pack mules. I think there are still some Nexus recasts of this set on the market. Atlantic gets a bad rap sometimes but I like them.