Chief Chilly

Chief Chilly is a variant of the Chill Bully fought in Super Mario 64 DS. They are fought at the end of the Chief Chilly Challenge level, and is usually fought with Luigi. When defeated, they will drop the key that unlocks Wario. They function similarly to a Chill Bully, running towards the player and attempting to knock the player off of the arena into the freezing water that will damage the player. If knocked into the water, they will break part of the arena and go back to their normal state. This must be done a total of 3 times to defeat Chief Chilly.

Super Mario 64 Manifestation
In some personalized copies of Super Mario 64, Chief Chilly, as well as Chief Chilly Challenge, can be found in the original Super Mario 64 game. It can usually function as a reskin of the Chill Bully, or be replaced entirely by a Chill Bully, though has been reported to be nearly identical to it's DS appearance. Upon being defeated, it will usually drop a star or key to unlock another castle floor.

Yoshi Battle
Despite the fact that it is impossible to reach Chief Chilly using Yoshi without cheats, Chief Chilly has specific dialogue that would have been used in a fight with Yoshi. There are 2 associated lines of dialogue: One for when you encounter Chie Chilly, "Hmm? I see you have no mustache. Poor, bald, little creature. It's not a fair fight for you, but luckily, I'm not a fair fighter. Let's go!" and another for defeating them. "I simply cannot believe that I lost to a hairless pip-squeak like you! My mustache was my only joy. Now what am I going to do?" Some people have theorized that Chief Chilly could have been fought using Yoshi's Power Flower ability, using the fire breath to melt Chief Chilly.

In some personalized copies of Super Mario 64 DS, the player can actually fight Chief Chilly with Yoshi, typically alongside a repositioning of Chief Chilly Challenge to allow for such an event to occur. As one would expect, the fight most commonly requires the Power Flower which can randomly spawn around the stage, but some say that Chief Chilly can drop blocks of ice that Yoshi can eat and make into eggs to knock Chief Chilly into the freezing water.

Trivia

 * In terms of appearance and function, it can be said Chief Chilly is an older and possibly stronger version of the Chill Bully.