Here’s some of my own:
1. “Aegislash” is pronounced based on the famous sword called Aegis (a word introduced to many people through the Xenoblade Chronicles series) with a soft G sound as opposed to a hard G. I can’t tell you how long I had this one messed up.
2. Helioptile’s (and by extension Heliolisk) name may be a reference to a god from... which mythology was Helios again? In any case, the “Helio” sound can be pronounced in multiple ways, like “heel-ee-oh” or similar to the pronunciation of “helicopter”.
3. If you’re ever in a bad mood and want to get on someone’s nerves, just look at Chimecho and pronounce it as “Chime-cho” instead of “Chime-echo” the next time you’re talking about the Pokémon.
4. I can think of at least three ways to pronounce the “Vika” sound in “Vikavolt”. Most people from what I remember defaulted to “Vee-ka” similar to Pikachu, having already been familiarized with the “Ika” portion. It can also be pronounced as “Vik-uh” or “Vike-ah”, to name the ones I can think of.
5. We’ve all heard the debate about the C in Arceus’s name at this point, but I just recently found out that there’s another error we’ve possibly been making this whole time. The last few syllables have historically been pronounced as “Ee-us” just like the spelling implies, but that part of Arceus’s name may also be referring words like “Deus” or “Zeus”, the most famous of the Greek gods, to name a few. This is notably pronounced in different ways depending on cultural factors. So when you hear a certain YouTuber pronounce it as “Ar-koos”... he’s actually not that far off.