Raikou and Rhyperior are still in C right now.
Anyways, C- noms.
Gastrodon: Only decent defensive stats, very passive, doesn't even check Rain because of Ludicolo.
Hariyama: Outclassed by Conkeldurr most of the time, very poor defensive stats off-set its large HP, not a very good Fake Out user.
Kyurem: Outclassed by Kyurem-B most of the time, really only has a slightly better special attack stat.
Manaphy: Really hard to fit on Rain without stacking weaknesses or making it generally worse, and outside of Rain it's really only an average sweeper due to its poor Speed. Also no Muddy Water sucks.
Medicham: So many better Megas usually. Mega Khan is generally better for Fake Out, while Mega Mawile is a better powerhouse. It fits somewhere inbetween I guess, but I'd still choose Khan over this almost every time.
Musharna: The reasons to use this over Cress are so minimal I wonder if this is even C- material really.
Slowbro: Slowking's better SpD is usually better than Slowbro's Defense