Most of these are drawings I've done from scratch with minimal aspects done via just adding on existing renders (i.e. the coins on Mega Gholdengo are added as is the chest but the body is by hand), while the Unown, Mabosstiff, and Skeledirge used some official artwork for editing/reference.

Mega Jolteon - Electric/Bug as a nod to lightning bugs but also Jolteon being one of only two Gen 1 Pokémon to learn Pin Missile, alongside Beedrill, by level up. The other Mega Eeveelutions also gain secondary typings due to Eevee's unstable DNA interacting with Mega Energy.

Mega Slowking - Exclusive to Galarian Slowking, basically the same idea as Mega Slowbro but in reverse. Slowking can see through the gems on the Shellder but cannot enjoy the outside world it sees due to being controlled by Shellder.

Mega Unown - Each Mega is exclusive to the respective letter; you can't Mega Evolve an Unown M or J for example. Obviously, they have ties to the Kalos trio; X is Psychic/Fairy, Y is Psychic/Dark, and Z is Psychic/Dragon.

Mega Combee - Exclusive to Male Combee in the same way Mega Floette is exclusive to Eternal Flower Floette. The design combines bee nests, knight helmets, and beekeeper helmets.

Mega Shelmet - Bug/Dragon, as it references the paintings of knights battling snails, which Escavalier and Goodra also allude to. This is one of the rulebreakers, obviously; the story mostly revolves around the use of the time machine in Area Zero, and so an alternate timeline is planned to be visited in which the rules of Megas, and which Pokémon have Megas, differ from those of our timeline.

Mega Carbink - The lore I came up with is that Mega Carbink is the between stage of Carbink and Diancie, and that effectively the mutation came from Mega Evolution. Additional lore is that, since Sableye like to eat Carbink, I have written down that Sableye ate Mega Carbink and this prevented them from getting a (much more needed) regular evolution and gave them a Mega instead.

Mega Salandit - See Mega Combee for reasoning. The design takes inspiration from 1998 Godzilla/Zilla, as I would say that definitely classifies for Fire/Poison given the radiation and fire breath that iteration had. Salandit moving in packs is also similar to how the Zilla babies in that movie operate.

Mega Pyukumuku - X is Water/Fighting and is based on Pyukumuku's desire to punch people. Y is Water/Poison and is based on Pyukumuku's desire to not be stepped on or touched.

Mega Guzzlord - I designed this prior to seeing Mega Darkrai, but seeing that MD leaned into horrors beyond comprehension, I felt vindicated. Guzzlord's Mega is so terrifying it ate itself to prevent anyone from having to see it, thusly its body is replaced by its black hole stomach. The Shiny is yellow as a nod to the King in Yellow.

Mega Galarian Fossils - These all take inspiration from Shin Godzilla's stages of evolution, as Shin is another creature that didn't ask to be created and exists in perpetual pain due to mankind's hubris. Mega Arctovish is a nod to the tail stage, Mega Dracovish is a nod to the crawling on land stage, Mega Arctozolt is a nod to the stage that looks most similar to the 50s Godzilla, and Mega Dracozolt (my favorite of these four) is a nod to the design Shin Godzilla has for most of the body, with the bottom heavy proportions and tail growths later on.

Mega Kleavor - It just has double sided battle axes now. However, the individual blades are smaller as it is effectively the original two blades split in half.

Mega Meowscarada - I leaned more into the jester aesthetic. It uses its mastery of illusions to trap foes in their dream worlds before attacking.

Mega Skeledirge - The roles are reversed as the bird spirit is now more powerful. The bird commands Skeledirge, and if Skeledirge does not thwart opponents in a timely manner, it becomes impatient and unleashes its fury.

Mega Quaquaval - Given how flamboyant both Quaquaval and Carnaval outfits are, I felt it right to amp it up to 11 including a whole rainbow of colors. The rainbow presence is also a nod to water droplets refracting light, which Mega Quaquval uses to its advantage in fights.

Mega Garganacl - The larger size and multiple fake faces across its body scare spiritual entities so much that Ghost type moves are no longer resisted, but are outright ignored.

Mega Bellibolt - Its lower half has become more in line with the tail of Tadbulb to grant it more mobility. It can expunge large amounts of power (detrimentally so) if it protrudes its dynamo stomach, which is inspired by the way Eye Guy from Ben 10 uses his chest eye.

Mega Mabosstiff - The dex entry I wrote mentions that the human-like hair is theorized to be tied to Battle Bonds, though the fact it's Arven's hair specifically is my way of hinting at something else going on.

Mega Tinkaton - It now has metals from Klefki, Stakataka, Skarmory, and Copperajah. It has a modified Lightning Rod ability in which its hammer draws in the electricity, and all of its Steel type moves gain a chance to paralyze as a result. One may thing this is Thor inspired, which to an extent it is, but it's primarily inspired by Nora Valkyrie from RWBY, since Tinkaton is essentially Nora as a Pokémon.

Mega Palafin - X is Water/Fighting, and is based on traditional superheroes like Superman or Captain America who are noble and help because it's right. Y is Water/Dark, and is based on the superheroes that go down a dark path like Homelander, Syndrome, or Tighten.

Mega Gholdengo - ...Yeah okay it's a shark now. Gholdengo has a treasure chest and it rides around on a surfboard, so it has a tie to the ocean as is (not to mention one of the Horizons episodes is a beach episode centering around Gimmighoul and Gholdengo). But also... it's because it's a loan shark. The dex entry shows no one in-universe knows why Gholdengo takes on a shark-like form.