Gengar: Doppelgengar | Gengar is a shadow and his name would be based off of doppelganger
Pelipper: Wayne | pun on Rain
Ferrothorn: Iron Barbie Girl | pun with Iron Barbs, and shiny Ferrothorn is pink
Swampert: Do you even liek? | because of the "I herd you liek Mudkipz" hype + pun on Do you even lift?
Mega-Swampert: Bulk Biceps | just look at those arms
Greninja: Professor Croak | Pun on Prof. Oak + Croak
Gastrodon: Teddy West Side | nickname that Ted from How I Met Your Mother gave himself + West Side Gastrodon
Diancie: Carbink-EX | basically a Carbink on steroids
Zygarde: Severus | Snape's first name (Harry Potter) + Slytherin's house symbol was a green snake
Necrozma: Edgelord | because of all the edgyness of the design + being a legendary
Mew: Mewcifer | Pun on Lucifer
Kingdra: Super Soaker | Based on design
Mamoswine: Donald Tusk | Name of the president of the European Council, which has Tusk in the name
Shedinja: Ghost in the shell | Name of a movie with Scarlett Johansson + description of what Shedinja actually is
Mega-Metagross: Weapon-X | because of its insane power and his design resembling an X
Roserade: Mrs. Bouquet | after the british sitcom Keeping Up Appearances + bouquets for hands. Hyacinth Bouquet also works
Bronzong: Big Ben | Super obvious, super fitting
Chansey: Give peace a Chans | Pun on Chansey. Plus it's known for its role on stall teams, which don't tend to inflict a lot of direct damage
Rhyperior: Rhype Train! | Pun on Hype Train. And it hits like a train
Porygon-Z: EZP-Z | Pun on EZPZ and P-Z, common abreviation for its name
Gallade: Seto Kaiba | a main character from YuGiOh which has a strong resemblance because of the cape and the blade arms
Stoutland: DawgsingMCHN | Pun with dawg (dog) and dowsingMCHN, the name for the Itemfinder, which is what Stoutland does in Sun/Moon
Excadrill: Excerminator | Pun with Excadrill's name and exterminator
Amoonguss: FunGuy | Pun on fungi
Aegislash: Heartsbane / Oathkeeper | Both famous swords from the Game of Thrones books and series
Lurantis: Not Leavanny | It looks a lot like Leavanny, but it isn't
Golisopod: Brave Sir Robin | a character from Monthy Pyton's The Holy Grail, who has a song about him bravely running away. Much like his ability Emergency Exit
Rowlet, Dartrix, Decidueye: Doctor Hoo | Pun on Doctor Who + Owl sounds. Dartrix also has a lot of resemblances to Matt Smith, the 11th Doctor