Infected Brute Chieftain

The Flood Brute Chieftain is a glitch that can be found on the Halo 3 level, The Covenant.

Walkthrough
It can be done by getting a Flood Infection Form onto a Brute Chieftain. These two do not normally exist in the same area. The only way to get them together is to kill all your allies outside the third tower in the level, while leaving some Flood Carrier Forms alive. Kill the carriers and let the Infection forms chase you up into the snow area. Kill the Brute Chieftain there and let the Flood take him over.

After being infected, the Chieftain's appearance does not change except for some Flood infection on his back. He retains all his normal combat abilities, including vehicle boarding. His shields stay down. The infected chieftain attacks both the player and the Covenant nearby.

Explanation
The infected Brute Chieftain may be an old version of combat forms created in-game. The Flood would not totally change the host's body immediately after infection. This idea would have been cut from the game, possibly because the combat forms looked too similar to the original enemy.