You must delete the META-INF folder inside minecraft.jar to prevent the game from crashing.
Move the .class files from the extracted nycat folder into the minecraft.jar . nycat.rar
The pixel art for the Nyan Cat entity and its rainbow trail. How to Install the Nyan Cat Mod You must delete the META-INF folder inside minecraft
Compressed archives are a common way to hide malware. If you downloaded nycat.rar from an untrusted site, follow these steps: WinRAR vulnerability exploited by two different groups nycat.rar