If someone were to have a patent for...let's say for argument sake..dog food, and has a myriad of patents that include all types of dogs, but you realize that the product is just as good for giraffes, and that there is a huge, untapped giraffe food market, can one obtain a patent for use in the giraffe food market, even though you are using the same, patented product because the dog food company never listed giraffes in their patent?
This question is basically: can a new use for a known substance be patented?
The answer depends on whether the claim to the new use is novel and non-obvious (as with all patent claims). In the example case, I can see three types of claims which might be used.
A giraffe food product comprising the known dog food.
This is not novel, since the known dog food would inherently be the claimed giraffe food product. The substance itself is therefore known. It could therefore not be patented.
A giraffe food product comprising the known dog food in a leaf shape.
Here, the substance is provided in a way that is uniquely suited to giraffes (since giraffes only eat leaf-shaped food). This is novel, because the previous dog food was just an unshapely mass. It is non-obvious, since dogs would refuse to eat leaf-shaped food, so it is not an obvious modification to make. So such a claim would probably be patentable.
A method of providing food to a giraffe comprising administering the known dog food to the giraffe.
Here, you are claiming something which has not been done before: nobody has fed the dog food to a giraffe. It is therefore novel. Moreover, it is non-obvious, because nobody would expect a giraffe to eat dog food. So arguably this claim would be patentable.