OGRE

The Object-Oriented Graphics Rendering Engine, also known simply as OGRE, is a scene-oriented, real-time, open-source, 3D rendering engine that is developed by the OGRE Team. It was used by Roblox until March 28, 2014, when it was replaced by a custom-built rendering engine. Its replacement was due to the engine beginning to hit its limits on low-end hardware and mobile platforms.

Trivia

 * Before its removal, the name of the engine was compared to a legendary monster of the same name.