Did you notice how certain modded biomes only have the same few pokemon? Or how some of them have no pokemons at all? This is because pokemon only spawn in compatible biomes (and blocks) specifically mentioned in pixelmon's config files. It's even worse for 1.12.2, as pretty much the only compatible biome mod is Biomes O' Plenty.

This is a modified version of BetterSpawnerConfig.json to make pixelmon compatible with a few more mods (17 mods in total). This way you'll get regular pokemon spawning in all biomes covered by this config. Some mods didn't have any pokemons at all due to unsupported blocks (Defiled Lands for example), this was also mostly covered for the mods that are included in this project, but I didn't include all possible blocks for all biomes. Instead I focused on the most common blocks for the sake of brevity. That said, all mod biomes are covered, and some mods have pretty much all their blocks covered (Underground Biomes and Defiled Lands, for example).

Pokemons might not spawn in modded dimensions though (such as Twilight Forest and Aether). I haven't tested this specific functionality quite fully yet. But for the overworld, nether and end biomes it works just fine.

The biomes were put in their respective categories, so Thaumcraft's Magical Forest is on the Magical category, for example. Otakomod's Blue Plains on Plains category, all Defiled Lands biomes on Evil, and so on.

Aether is not listed as a biome because I thought it would be weird for pokemon to spawn there, but if you want it added I can explain to you how it is done.

It adds biome compatibility with:

Twilight Forest
Blue Skies
Good Night's Sleep
Thaumic Augmentation
Thaumcraft
Defiled Lands
JEID's Error Biome
OtakoMod
The Aurorian
Mahou Tsukai's RealityMarble biome
Lands of Icaria
NetherEx
BuildCraft's 2 oil biomes
Stygian End

It also adds block compatibility (so pokemons can spawn) with:

Underground Biomes (all Stones and Sands)
Tinker Construct (Slimedirt, Brownstone)
Twilight Forest (Maze Stone)
Thaumic Augmentation (stone)
Quark (Limestone, Basalt and Marble)
Lands of Icaria (Aristone, Stone, Cobblestone)
Blue Skies (Turquoise Stone and Dirt, Lunar Stone and Dirt)
Good Night's Sleep (Dream Dirt, Delusion Stone)
Defiled Lands (Stone, Dirt, Sand, Grass, Sandstone, Gravel)
Aether (Holystone, Aether Dirt)
The Aurorian (Dirt, Stone, Cobblestone)

(note that not all blocks were included for the sake of brevity, this should cover most non-structure and non-player created areas)

So the list of all mods included in one way or another is:

HOW TO INSTALL?

First enable BetterSpawner on pixelmon.hocon file (useBetterSpawnerConfig=true), which is located on minecraft/config. Then all you need to do is replace the generated BetterSpawnerConfig.json file with the provided one (simply extract the downloaded .zip to get the file). Pixelmon's BetterSpawner.json file is located in minecraft/config/pixelmon (it will only be generated after you set useBetterSpawnerConfig to true).

I can also teach you how to do this for other mods you want, it's pretty simple.

This config file doesn't remove anything from the original nor does it modify any configuration other than biomes and land blocks.

PS: It might work on other versions as long as they have a BetterSpawnerConfig.json to modify, but no promises. Tested on the latest pixelmon version for 1.12.2.

Additional biome and block compatibility for Pixelmon