The large rough green inedible fruit of the Bois D'Arc or Osage-orange tree. No, horses do not eat them. Nothing eats them. They've even been used as insecticide. And the fresh sap in them is nearly impossible to get off of anything it gets on.
They're called horse apples because they're round and the same shade of green as fresh horse shit.
They exist for kids to throw at each other and raise big welts.
What the hell is this thing? It looks like a green softball.