While talking to Fluttershy and Pinkie Pie about their respective pets, Rarity mentions some strange things that her precious Opalescence has been doing as of late. She is hoping that Fluttershy, as an animal expert, can offer advice.
As it turns out, Fluttershy does indeed know what the problem is — but it's not good.
AUTHOR'S NOTE: Remember that 'pussy' is also short for 'pussycat.' Also, comments contain spoilers.