I'm not even joking. Three months ago, Greninja was ubiquitous as hell, and Starmie was completely gone (#63 in usage in December). Then Greninja gets banned, and suddenly everyone who used it replaced it with Starmie. Both are fast, decently powerful Water-types, so it goes without saying. Anyway, Starmie ranked #13 for the month of March. It was used no less than SIX TIMES AS MUCH as it was in December.
As a side note, Tornadus-T and Hippowdon are two Pokémon that have seen a huge resurgence in recent months - expect them to rise to OU in the next major tier update. The most recent quick update saw four Pokémon rise from various BL tiers to the tier above, including Serperior which is now OU. I have to say, it may have been a wise call to not release it last gen. On the other hand, Chansey usage is dropping like a ten-ton anvil. If it's unleashed into UU by the end of the year, this is going to be so entertaining.
Random thought: it's funny how people say Landorus-I is damn near broken, yet the Therian form is still seen three times as much. I mean, it's better than the six-to-one ratio from three months ago, but still.