Not to be confused with the Minipet Costume.The Chibi Costume is a retired costume that was a part of the Chibi Account Upgrade from June 2014. When you put this on your pet, they would be a relatively cuter and smaller version of themselves. This costume is aqua with a white swirl on it, a pink ribbon-like mist and yellow stars around it.
The Chibi costume is based on the Japanese slang word chibi (ちび or チビ), meaning someone or something that is shorter than all the others. It comes from the verb 禿びる (chibiru), which means "to wear out and become shorter," as shown with one of the Chibi Pet Potions to the right. The word has gained popularity among manga and anime fans.You can see a Chibi Chibs creeping up from the treasure chest in the Account Upgrades image.