I have a mission to find "Small Pack Horses" and regular "Pack Horses" don't count for it and I can't find any small pack horses for sale. I wonder if this was an oversight by the modder.

Yes, I also had the bug of being unable to find "Small Pack Horse" anywhere. Must be a bug.

If it helps I get al lot of horses from battle from nobles and doing the suspicious merchant which you get all the goods their selling

I can't find small pack horses either. Hopefully someone will reply with more information in the future!

i found boatloads of them in the north and central regions, its a matter of looking in the right places