Longhorn was originally the codename for the Windows OS that later became known as Windows Vista. This Longhorn-tan had several different designs, all of them having either horns or appearing as pigtailed lolitas with hairties in the shape and style of the Longhorn logo. Since becoming Windows Vista, no more fanart of these Longhorn-tans have been made recently.
There is another different Windows OS codenamed Longhorn (expected to be released as Windows Server 2008 or 2009) that is represented as a half girl-half mackerel character like Server 2003 Saba-tan and Server 2000-tan but yellow-colored, has curly blonde hair and a white crown. She is almost always seen with the other two Windows Server-tans.