Why would anyone have leftover antibiotics? I never get more than what I'm supposed to finish. Now left over pain killers yes. You have to be crazy to throw away pain killers, especially if you live on a farm.
There are a variety of antibiotics etc. you can buy online for animals that you would need a prescription for if you saw the Doc..
We've got quite a few antibiotics on hand in the fridge now, ready when needed.