SPOILERS. DO NOT LOOK BEYOND THIS POINT IF YOU DO NOT WISH TO BE SPOILED.

They are known as The Cobra, and there are five of them.

The Pain:
http://members.lycos.co.uk/hellointernet/The_pain_sm.jpg
The Fear:
http://members.lycos.co.uk/hellointernet/The_fury_sm.jpg
The Fury:
http://members.lycos.co.uk/hellointernet/The_fear_sm.jpg
The Sorrow:
http://members.lycos.co.uk/hellointernet/The_sorrow_sm.jpg
The End:
http://members.lycos.co.uk/hellointernet/The_End_sm.jpg

Also, check out the Unofficial MGS3 Website for background information on the bosses. (NOTE: BIOS CONTAIN STORY SPOILERS. READ AT YOUR OWN CHOOSING).
http://www.gamersbg.com/mgs/mgs3/characters.htm

One Final Note: Right now, I am not sure if I have The Fear and The Fury mixed up. Once I found out which character is which, I will let you guys now. For now, enjoy them, hate them, discuss them, compare them, whatever.