Pretty self explanatory. What ponies did you used to dislike but now love, and what ponies did you like at first but now hate/dislike?
Here are some of mine...
Dislike to Like/Grew on me:
Rarity: I used to think she was shallow and quite rude to her sister, but now I see she's a lot nicer than she lets on and can be considerate of others, especially her friends.
Shining Armor: I used to hate how perfect he was, he had practically no flaws until S3 made me like him as a cool big brother. I also like his relationship with Cadence and think he's kind of handsome.
Rainbow Dash: It's not that I didn't like her, but she really really grew on me. Especially in late Season 1 and Season 2 when she's a well meaning big sister to the other mane six. And then she's amazing in Flight to the Finish being almost like Pinkie Pie. All that character development made her my favourite pony.
Like to Dislike:
Trixie: I used to think she was an interesting character at first and a good villain until "Magic Duel"'s "redemption". I felt her redemption rang hollow and wasn't a true redemption. Plus she was really mean in that episode.
Brad Sentry: I thought he'd be an interesting love interest with a lot of character until I realized EQG gave him no personality other than being nice. He doesn't have any noticeable flaws either, which makes me think of Shining Armor before he grew on me.
Diamond Tiara: Until Flight to the Finish, I didn't think she was that bad. I thought she had a lot of flaws but she at least cared for Silver Spoon so that's got to mean something right? Wrong!