ANI, Windows Animated Cursor (.ani)
The ANI System file format
ANI is the file extension most commonly associated with animated cursor icons on Windows computers. Cursors that have animation (like the spinning circle) are saved in this format. Custom cursors for usage on Windows operating systems can also be saved as ANI files. They are often part of custom desktop themes for windows or can be downloaded on certain websites. There are several programs that allow to create own animated cursors as well. On Windows versions from Windows 2000 up to Windows Vista, there was a reported security issue regarding ANI files. A security update for these operating systems is available and the issue hasn't been reported for newer Windows versions.
Technical details of ANI files
ANI files are comparable to CUR files. However, while CUR files contain only one image, ANI files comprise of a collection multiple images (ICO). ANI files are based on Microsoft's RIFF format which allows the storage of individual frames within one container file format.