Some of the readers might have heard of a game called Duke Nukem 3D. Released in 1996, it was one of the first 3D games I had the chance to play. An interesting feature of that game is that most of the interactive elements (including the enemies) were not actually 3D. They were 2D sprites rendered on quads which are always facing the camera (below).
This technique is called billboarding, and early 3D games were using it extensively. Even today it is still used for some background details, such as trees in a forest far away. For instance, one of them is Massive Vegetation, which uses billboarding to render grass blades in a very realistic way.