Trubbish and Garbodor: At first I was like, "Eeewwww, it's a trash bag. Unoriginality blahblahblah" but then it dawned on me that they're kinda ugly-cute.
Archeops: It looked cool but I wasn't planning on using it until I got my own and dat Attack stat and Acrobatics and you know the rest.
Chimchar, Monferno, and Infernape: Not a big fan of monkeys but after PMD and the anime they're one of my favorite starter families. Also, Infernape is one of the few starters that is great in the metagame.
I hated him at first and I caught one for my dex and named him F**k then realeased it.
I ended up renting one for the PWT and he was pretty handy. When an opponment would hit him we would loose defense but he would gain speed and when he was about to faint he would use explosion.
Aand when i first came to the fandom I thought all the snakes and ghosts were ugly. I dont hate any Pokémon now. They all have something good about them
Tangela. I don't know when it started, or why, but Tangela has risen from the depths of obscurity to being one of my favorite pokemon of all time. I never thought about him much for a long time, but when I tried to narrow down pokemon that I wanted to use, he was consistently near the top of my list! And then 4th gen came out and I discovered that he had an evolution, but Tangrowth has never appealed to me as much.
When I was playing Pokemon Emerald, I desperately wanted to catch a Grass Pokemon, and the only one available was Cacnea. "A cactus? Really?" An he had really weak moves too - Absorb? Leech Seed? Thanks a lot... But later he evolved into Cacturne, which is kinda cool-looking, and he finally learned some really useful moves. He would certainly be in my hall of fame, but before one o the final battles my save file crashed. Oh well, maybe another time...
Breloom: Because I don't like Grass-types and because I found its design rather dull at the beginning. After battling one whose moveset I was impressed by, I decided to train one by myself, and now it's one of my team's powerhouses.
Heracross: Basically the same as Breloom; I don't like Bug-types either.
Galvantula: The same reason as Heracross. It's weird because I don't like Bug-types, but the very moment I saw Volcarona I just thought "That thing has to be mine".
Rotom. At first I thought that its concept of inhabiting different household appliances was stupid, but once I actually used it and started playing around with its forms, I realized how awesome it is. Rotom is now one of my favorite pokemon.
Togekiss. My friend told me about Togekiss and said it was an awesome edition to her team, so when I restarted HeartGold, I decided to give it a chance. I loved Togekiss after that and i will always have a Togekiss as my Flying type in HeartGold now.
There isn't much I expected to not like, to be honest. I kinda start off not caring about/disliking anything I don't already immediately fall in love with. And typically, my opinion is easily swayed into liking things more once I get to know them, so to speak. I suppose I can say I slightly didn't expect to like Octillery. After using it in battle, it proved to me it was a good Pokemon with a really fun moveset. And the design did grow on me. (Though I didn't quite hate it before, to be honest. It was just like any other Pokemon to me.)