Medications as packaged for livestock are not as quality controlled as for humans. Additionally they are packaged in different doses then for humans Especially if you are trying to use something forumlated for a cow or horse on a human. You would have to calcuate the formulation differences between a 1000 lb animal and a 100-200 lb human. Any kind of calcuation error could lead to an overdose and possible death of the human.
If you are talking about things like Ointments and linaments, those are fine, but any injectable, or oral medications that have to go through the body are not suitable to mix the two between livestock and humans. The only time I'd mix the two are when I only had human formulated stuff to give to a livestock but never the other way around.