Hi Paul
Dixons do both oxen and mules in their Wild West range:
http://www.dixon-minis.com/search/oxen/http://www.dixon-minis.com/search/mule/Pegasus also do a 1/48 set that includes 4 oxen (eight or nine quid on eBay) (it also includes a plough, and a wagon bed that could be the basis of a scratch-build):
Pegasus Hobbies 1/48 California Mission Indians Set No.2 # 7005
They also have a couple of mules in their 1/48 California Gold Miners # 7007 but I can't see that these are great value (you only get two mules per pack).
Other mule makers include:
https://www.forlornhopegames.com/index.php/product/2107-pack-mule/category_pathway-105I think the Iron Duke oxen may have the 'wrong horns' for European oxen - they were designed for India, after all.
My reading of the logistics problems suffered by the Allied Army is that transport was a constant problem. There were simply insufficient horses for the basis needs of the army with the result that artillery and supplies were carried by mule trains / mule and oxen drawn wagons. These had civilian drivers who often legged it along with their mules at the first opportunity.
"The gun bullocks now grazing at and in the neighbourhood of Celorico to go to Almeida as soon as possible to draw the guns from Almeida to Gallegos" Memo from Wellington to Commissariat General Bissett 01/01/1812
Now it may be that oxen were mostly used for hauling the siege train but I believe this was done by the Spanish for their foot artillery too. I think it was a case of use whatever was available.