Jackrabbits. They are as hard to put down as a buffalo and harder to hit. After jackrabbits, a buffalo will be a cake walk. I'm not joking about being hard to put down either. I've many times seen them hit with everything from a 22 LR to a 455 in a hand gun and all manner of center fire rifles and keep right on running.

Here where I am we have only cotton tails which are less likely to run and very easy to put down. But I hunt them with a 375 H&H. I use a hard cast bullet which just drills a hole through them and doesn't spoil meat.