The Yellow Neck Cassowary is a special target (rare animal) that is your prey for the Sniper Rifle Hunt: Yellow Neck Cassowary mission in the Path of the Hunter missions. Remember that Cassowary birds in Far Cry 3 are aggressive and will attack on sight.
Where to find the Yellow Neck Cassowary: This creature is on the island with the "Temple of Stone" which lies north of the Valsa Docks outpost. Cassowaries attack when shot at, so find yourself a safe spot. For example, the area on top of the nearby bunker is a nice camping spot.
Skin the bird to receive Yellow Neck Cassowary Leather, which can be used to craft the top Fuel Sling upgrade, the Extended Fuel Sling.