Honestly it just depends on what first comes into my head. Sometimes it's a JoJo reference or sometimes it's just a random word that could pertain to that Pokemon's type. I named my Marowak in Let's Go: Eevee, which could both be a jojo reference and a musical reference.

Most of the time, it ends up being something random that just comes to my head. Whether it's a terrible pun, a random regular name, or something else, it's a nickname. My friends mostly do the same thing, although they usually lean more towards the "terrible pun" or "something else" side of the scale a bit more than I do.

I prefer to not nickname my pokemon simply because I'm not great with nicknaming things and I would just end up spending a long time obsessing over just the right name to give to each pokemon. In the end, it's just easier for me to leave them with their original names, plus I spend a lot of time trading pokemon with others, so out of courtesy for them, I just leave their names alone. Too many times I've received amazing, and sometimes even shiny pokemon, with really dumb nicknames and it just deflates a lot of the excitement for me, and I can imagine many people out there feel the same way.

That said, the only pokemon I nickname is Magikarp. Every time I hatch one or catch one that I train or use in a playthrough, I nickname them "useless" until they evolve into a Gyarados and become useful. After becoming useful, they earn the right to be called Gyarados. It makes them train harder and become stronger, I promise.

I don't usually give my Pokemon nicknames, but one that I've always thought was funny was the nickname I came up with for my Slakoth in Pokemon Emerald. I named him "Bread" because he was always loafing around. Hahaha I'm so funny.

I named my nidoking Littlefoot, after the character from The Land before Time. Just because I couldn't really think of any other nicknames. I also named my pidgeot Giorno.

I name most of mine after regular human names. I only play nuzlockes so I get stuck with stuff you wouldn't normally use.

My favorite ones so far are George the Grumpig, Wario the Probopass and Stefan the Clefable

i nicknamed my rattata ratty 

I actually just remembered one that I recently used because I couldn't be bothered to think of a good one and I found it so funny (it was 5 am) that it's now a permanent nickname.

Swim the Blastoise. I can't get over how seeing that name made me laugh. Probably because it's so short and stupid.

Well, whenever I get a fire type starter, I name it Omegaflare. The only other time I've given any thought to a nickname was for Azumarill and Golem. In different games, I've given them both the name Pinball.

I've always enjoyed nicknaming my pokemon, it just always felt right because you have a certain nickname for your friends or your SO, or pets. Some of course I've always kept their normal names, but Leafeon would always be Leif, after the viking. Charmander would be Kaiser, after a user on this site. All my pokemon has certain names because of certain people who reminded me of them. I'm super sentimental 

I don't nickname Pokemon in my first playthroughs of games, it never felt right to me since I'd like to experience what the actual name of a Pokemon is when I first see it, rather than nicknaming a pre-evolution and basically only seeing the name once when it evolves, when it says " Your (insert nickname) has evolved into Simisear! or something like that. Though when it comes to challenge runs like nuzlockes and such, I definitely like doing that since it feels more personal when you lose the Pokemon during the challenge.

Because I legitimately play through Pokémon games multiple times, you'd thing it would be hard for me to nickname my pokemon, but not really. I end up going with some kind of a naming theme. Like maybe my favorite anime characters from various anime's or just characters from the same anime. Or maybe they have a y in their name. Or a Japanese translation of a word associated with said Pokémon. I also have a very odd thing for naming female lapras' Lady M and I don't know why,

I almost never nickname my Pokèmon. I prefer to see their names change as they evolve. It makes it easier to remember new ones.

When I was a kid though...I had a Charizard named GOAT, and I was named BIKE. I think that was supposed to be a joke, very strange times those were.