Sandi asked: "What's the opposite of a plant eating sheep" to which the correct answer was "A sheep eating plant." This is actually not the opposite but rather the contrapositive. The opposite of "A plant eating sheep" is "A plant not eating sheep"
And I say the opposite of "a plant-eating sheep" (note the hyphen) is "a sheep-eating plant"!
The term contrapositive refers to statements of the form "if P then Q", i.e. given the statement "if P then Q", its contrapositive is "if not Q, then not P". A phrase like "a sheep eating plant" is not a statement; it's just a phrase, therefore it makes no sense to apply the concept of contraposition to it.