Before X and Y were released, a Pokemon that started to grow me more was Ampharos. From seeing its fabulous Megavolution, I felt I had to had one. Therefore, I trained a Mareep I got from the Dreamworld on my Pokemon Black into an Ampharos. However, I don't know if I want to transfer it to Y as I already have an Ampharos there.
As I have been playing Pokemon Y, two Pokemon that didn't expect to like so much are Aromatisse and the Goomy line. Before, when I saw Spritzee and Aromatisse through the news, I thought they were nice-looking Pokes. But now after using one, I really appreciate the line. I'm captivated by Aromatisse's looks, cry, and moveset. As for the Goomy line, I was kind of spoiled of its image here on Pokecommunity so I wonder what kind of Pokemon it was. It turned out it was a Dragon type but it just looks so derpy that I had to train one!
Along with those, there are Pokemon that I've trained on Pokemon Y that never got to use on my other games like Scrafty, Toxicroak, and Crustle.