There are three dog heads on the wall that once you kill a zombie next to that, dog head will spawn. Feed him 6 to 10 zombies. If you play Black Ops 2 Mob Of the Dead. This is almost the same as how you will get the Tomahawk the only difference is the location.

First Dog Location, Cell Block 2nd Floor.

Second, New Industries.

Third and last, Eagle Plaza.

To get the tomahawk, you need to go to Underground Passages. You can go true by shower or on Warden’s House either way.