Westwood included several 'special units' in Renegade Multiplayer Mode - this page explains how to access them and also has sub-sections depicting and describing the units themselves.
The units may only be accessed in non-ladder games; otherwise they will not work.
To activate the special units, press F8 and then type "extras fnkqrrm". Then, when at a purchase terminal menu, hold "alt" and click on the characters or vehicles screen to display the secret units.
Several units are unlocked for both GDI & NOD. The below links lead to pages containing statistics, comments and pictures of the units.