Pokey

Pokeys are enemies that appear in Shifting Sand Land in Mario 64. They are tall yellow cactuses with a head as the top segment. They have five yellow body segments that Mario can knock off. Their head is a slightly darker yellow with fewer spikes, and they feature orange eyes, ears, and a smiling mouth. They can be defeated by hitting, jumping on, or throwing a bomb-omb on their top segment. Their lower body segments reappear four seconds after Mario knocks them off. If their head is knocked off while their other segments remain intact, the other segments vanish instantly. When defeated, they drop a blue coin.

Headless Pokey
Some reports claim that a Pokey can appear without a head, making it much harder to defeat. It also reportedly moves much more erratically than the standard Pokey. What causes this anomaly is unknown, though it may be the A.I. attempting to make Pokeys more difficult by removing their weak point.