The Strider is the true main antagonist and final boss of the Half-Life sidescroller game Codename: Freeman (which acts as an alternative to the storyline of Half-Life 2). It is the cause of the 2D Dimension through unknown means. It is based off the Strider enemies from Half-Life 2.
Gordon Freeman finds himself fighting through a 2D world which is based off locations of Half-Life 2 and its beta (as Codename: Freeman was released prior to Half-Life 2 itself) such as the Docks, the Beach, Nova Prospekt and City 17. After fighting through many Combine and aliens, Gordon confronts the G-Man who tells Gordon he is not responsible for the missing dimension and more sinister powers are at play. Gordon later discovers the one behind it is a strider and fights off against it. When the Strider is defeated, Gordon returns to the third dimension.