Addition:
Roserade
kingdra deserve a mention?
Deletions:
Shoddy usage statistics back my claims up, but I'll post little snippets of why usage has decreased/increased and why it is no longer a threat.
Pokemon (Overall usage in January)
Rhyperior (55th) - We greatly overestimated this Pokemon. Usage declined almost immediately, and in the vast majority of cases if a team can counter Tyranitar and Garchomp, it can counter Rhyperior.
Raikou (62nd) - Usage declined due to tremendous Blissey usage and most players are opting out for the quicker setup power of nasty plot.
Tauros (108th) - Normal cbers are inadvertently walled by steel types intended to take Garchomp outrages
Staraptor (56th) - ^
Alakazam (51st) - Grail, due to the excess of of scarfers it goes down easily. i guess you could make a case for scarfzam being able to deal damage to a team, but people would much rather opt for scarfgar due to better resistances and hypnosis
Medicham (76th)
Aerodactyl (52nd)
Sceptile (74th) - I'm not 100% sure why these last 3 have dropped so much in usage. Medicham has that huge attack that reaches 720 with a cb slapped on and has ice punch to deal with Gliscor, the most common cb fighter counter. Aerodactyl can make for a reasonably good revenge killer and Sceptile is a subseeder with amazing speed that also has the ability to run specs or even swords dance sets off of reasonable sp. attack and attack stats. But even though these pokemon are usable, there's no point preparing for a pokemon you're not going to run into.
Slaking (127th) - normal type cber spiel
Additions:
Yanmega (33rd) - Can tear through any team that lacks a general special wall (ie blissey or snorlax)
Mamoswine (35th) - Ice/ground is a ridiculous stab type combination. People are starting to wise up to how good it is and usage is rising
Scizor (39th) - Technician on quick attack/aerial ace and ability to swords dance/agility pass or sweep itself make this a viable OU threat.
Additions-
Yanmega(33)-I have no battle experience with Yanmega on my side, but I do know that he rips through unprepared teams. Yes, Blissey walls this all day long, but that's why I say he rips through unprepared teams, not everyone uses Blissey. If you lack priority and have nothing else to wall this guy, you're not exactly in the best position possible.
Yanmega may have the ability to get fast and damage things a little bit with Tinted Lens but when a Pokemon only has 4 maybe 5 usable attacks, 3 of which are horrible offensively (Air Slash, Psychic, Bug Buzz), and has to resort to Hidden Powers to damage Pokemon it is in no way a threat. Hypnosis + Protect alone do not warrant its usability since it can't up its attack stat in any other form other than hold items. Stealth Rock seals the deal for this flying bug.
Magnezone(32)-I'm actually not sure about this guy. I have no battle experience with him on my side, so I can't say if he is a threat or just a threat to Steel, but a quick glimpse at him and he certainly isn't lacking Special Attack, and he has a pretty good STAB in T-bolt. I'm really just putting this guy out here for someone else to say something about.
No way does Magnezone deserve a mention. It should only stick to trapping Steels. It can only do so much with its very limited move pool. Fun to play with but nothing too serious.
Removals-
Tauros(108)-The usage speaks for itself, STAB on normal just doesn't cut it in this metagame.
Intimidate + Pursuit should be enough to keep this thing in the threat list. He also gets Earthquake and Stone Edge. STAB Return/Double-Edge deals a lot of damage to anything that doesn't happen to be Steel, Rock, or very Defensive.
Yeah, uh, tell that to Gyarados and physical Salamence... Porygon2 scares the crap out of those pokemon. Not to mention it stops Heatran, Jolteon and Vaporeon dead... I would call that a major defensive threat. It's not really scoring any OHKOs but that doesn't mean it isn't dangerous.
once again, where's Spiritomb, it was OU a while back, where is it?
This is an "offensive" threat list right? Therefore, I move that Blissey be removed from the list, as even though its usage remains huge, the CM attacking version remains quite rare compared to all the support versions.
Jolteon- This thing ties or outspeeds every standard Pokemon in the game yet it can not kill things thanks to its lackluster move pool. It has to resort to Hidden Powers to deal damage to Pokemon and thats not always a guarantee. When Baton Passing besides Pseudo phazing or Wish passing its only other viable option is Agility. This fragile pup is of little concern to the likes of the big dogs.