The best Game Development alternative to ZBrush is Armory 3D, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 50 alternatives to ZBrush and three of them are Game Development Tools so hopefully you can find a suitable replacement. Other interesting Game Development Tool alternatives to ZBrush are Zmodeler and Dead Deer.
Armory is an open-source 3D game engine with full Blender integration, turning it into a complete game development tool. The result is a unified workflow from start to finish, making you work faster. No more jumping between different applications.
Armory is a game engine not a sculpting software like Zbrush
ZModeler (or Zanoza Modeler) is a 3D modeling application. It is aimed at modelers who model for computer games such as 18 Wheels of Steel, Grand Theft Auto, Need for Speed, Midtown Madness or similar video games.
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Zbrush is for sculpting primarily on the other hand armory is a game engine.