Hello! I have a question and I hope someone can help!

So I am trying to make a birthday present for my friend, it's a horse bottle opener. So I have an STL file of a horse, and an STL file of the bottle opener I want to use, and I want to make them one part. So basically I want to position the bottle opener in the horse model and then merge them so the bottle opener stays intact and the horse gets cut away. So I know this sounds weird or easy, but the problem is that the bottle opener model has a slot in it to insert a quarter. When I overlay the two models that hole fills in. So I need a way to maintain the bottle opener.

I hope this all makes sense, I have accesses to autodesk, meshmixer and a few other programs.

Thanks a ton for the help!