According to Shape, you should be replacing your mascara every 2-3 months anyway so here's something you can do with your old mascara brushes. Donate them!
Savannah Trantham, co-founder of Appalachian Wildlife in Western North Carolina, posted on Facebook on Friday urging people to send her their old mascara brushes rather than tossing them out. Trantham and her staff upcycle the brushes to help remove fly eggs and larvae from the fur of the animals being rehabilitated at the refuge. [Source]