The cyberdemon has goat legs but one of them is metallic or perhaps robotic. His weapon is a rocket launcher that replace one of his hand. His tactic is to mindlessly shoot 3 rockets in a row at the player which deals tremendously high damages even at the lowest difficulty. However this is his weakness because a player with some experience will be able to dodge the rockets easily because of the simple attack pattern it uses.
In DOOM II, in the stage named GOTCHA!, this stage is meant to show a battle between a cyberdemon and the final boss of Ultimate DOOM, the spider mastermind. The cyberdemon usually always win unless the player interfer.
The cyberdemon makes a come-back in DOOM III where he is the final boss of the game. Despise his incredible damage rating, if the player defeats it the usual way, the cyberdemon is quite easy to vanquish.
The Cyberden and Go 2 it in Final DOOM plutonia is the stages with the most cyberdemons.
The Cyberdemon makes a return in Doom III, but apparently plays a very significant role in the game (no, you don't play as him, but he has a major purpose in the storyline). Also the Cyberdemon looks bigger & badder than his predecessors.