"Greenhorn (containing the sense of "new, fresh, recent") was first "young horned animal" (1455), then "recently enlisted soldier" (1650), then "any inexperienced person" (1682)."

The online etymology does not speak of slaughtered animals, only of 'new , fresh, recent. These words with horn at the back. I really do not understand where 'inkhorn' comes from. ('=')