If so, when? GB lists him as appearing in MGS3, but I can't remember. And Raikov isn't Raiden, right?

Selecting the option "I like MGS2" before starting a new game, this changes the opening. Snake is wearing a mask that looks identical to Raiden's face. It's kind of weak, but he's there in a way. If it were my decision, I would say he shouldn't be on the list.

There are a lot of references to him, so I can see why he's listed as in the game. Raikov is basically him. There is a Raiden mask that Snake wears, and I believe he's in the special features on Disc 2.

He is referenced and there is a Raiden "mask" as well as him being featured on the second disc in some of the parody videos but he isn't actually in the game as a character. I personally think he shouldn't be listed as being in the game because he's not really in the game at all. I don't think that references or a mask count.

Raikov was a total joke created because of all the people who hated him in the second game. I wish Kojima hadn't made people choose "I like MGS2" to see the easter-egg beginning, I would have loved to see everyone flip at the beginning... When I saw Snake take off that helmet and looking like Raiden I was like, "WHAT THE HELL?!" It was awesome! :D

If you mean the main story no. It was set before he was born. But I do have reason to beleive that he was in their as a joke or something