The Zombie model is the zombie model from the Left 4 Dead beta.
The Zombie model shares its animations with all other player models meaning it has no original animation.
Although not making any official appearances in the series or game, the Zombie model is often used by community custom servers for zombie-themed game modes.
However no additional information was given by the developers and Counter-Strike: Global Offensive has yet to see an official zombie-themed game mode. Said post informs that the developers are working with the team to make the fan made "Zombie Mod" more compatible with Counter-Strike: Global Offensive, as full mod compatibility was seen as an important pillar to the community. Zombies were first referenced in the Counter-Strike Blog post " Braaaiiiins!". Model was added during 8/20 Update along with Weapons Course.
The Zombie is cut or unreleased character found in the Counter-Strike: Global Offensive game files.