There are probably more, but I remember when ruby and sapphire first came out, I didn't know how to pronounce makuhita, namely. Excuse it's name was too long and I was too lazy to read it all and I was like 8 or 9 and I think I pronounced it mar-ku-ta, I didn't know it's proper name until it appeared in the anime. I knew I was pronouncing it wrong all along though, so now sure if that counts.
Ok I remembered more.
I pronounced jirachi as Ji-ra-kai
Raikou as rai-koo (not that huge a deal)
Most of the ones I have trouble with are 5th gen Pokemon. I was in japan a few weeks after 5th gen came out over there, so I bought a copy whilst I was there, and it really helped me learn the Japanese names of the 5th gen Pokemon, so when thppthey came out in English, as I'd already played it I basically just spammed A so I never really learned half the English names for fifth gen. Most of the time I have to think a little bit when I hear the English name because I'm not familiar with it.
Even when I have to type a fifth gen pokemons name, I almost always type it's Japanese name and then I'm like "o rite" and if it an remember it's English name I'll change it, if not then I'll have to google it.
But yeah, that's basically the only ones I have, I've always found my inability to pronounce 5th English names interesting tbh
Op waiti remembered another when I was reading through the thread.
I pronounced groudon as groo-don and kyogre as Kai-oga-ree and then when my friends were pronouncing them differently I was like "yea groodon is it's name before it's awakened, and when you awaken it it's called groudon, and it's called kaiogaree before it's awakened, and kyogre after it's been awakened" lol good times
Also think I used to pronounce meganium as meg-nanium and banette as ban-a-tee
O yea and ofc arceus as ar-see-us and regice as regi-ice (I still pronounce ipthese as ar-see-us and regi-ice coz yolo)