Just curious, are you paid per animal that you groom or a set amount per hour that you work? You don't have to answer that here, but I think it makes a difference.
If you are all paid per animal, then you are using your time making bows at home to maximize your income. By having the bows ready to go, you save time on the job and can groom more animals than if you took work time to make bows (any bows, plain or fancy).
When you give your bows away to your co-workers, you are giving away the advantage that making bows at home gives you--and you are given that advantage to your co-workers, to say nothing of the money you spend on ribbon and embellishments.
If you are all paid a set hour wage or salary, then you are artificially boosting your co-workers' productivity. If your employer expects everyone to make bows during work time with their ribbon, you are letting your co-workers work faster by providing them with pre-made bows. This might have an effect during performance reviews--they would come out looking like faster workers than they really are.
In your shoes, especially since you are spending your own money on the materials for the bows, I'd just stop giving them away. I would use the explanation that you don't have time to make more than your own supply of bows. If you argued that the bows cost you money for ribbons and such, they could always offer to pay you for the supplies. But it's harder for them to argue that you have more time. And if they argue that you have lots of bows at your work station, you can just tell them that you make a week's supply at a time, and those bows have to last you all week, and again, you are sorry, but you can't give them away any more.
There will probably be push-back for a few days. You will have to stand firm on this, because if you cave in after saying you won't give the bows away anymore, it will be twice as hard to attempt this all over again.
And you certainly aren't being petty. These are your bows that you spent your time and your money and your skills at bow-making on.
Your co-workers are pretty entitled, if they think you should just be giving the bows away.