Nether Soul Sand

Spawning data tags simply contain a list of Biomes. If you're referring to this list for a Pokemon, please double-check if it has an Anti Condition or Anti Biome. Anti Conditions and Anti Biomes are a way to essentially state: not in this biome/condition, so you'll need to cross-reference them. For instance, if a Pokemon is set to spawn in is_ocean, but has an anti condition of is_cold, it will not spawn in cold oceans.

Our Pokemon spawns are generally based on default Cobblemon configurations with additions to allow the entire dex to spawn in all Minecraft biomes and supported biome mods. Most biome mods are supported such as Terralith, Oh Biomes You'll Go, Incendium, Better Nether, Gardens of the Dead, and The Bumblezone. Other biome mods may also be compatible, provided they label their biomes with data tags.

Nether Soul Sand Biomes
minecraft:soul_sand_valley	betternether:soul_plain	betternether:wart_forest
betternether:wart_forest_edge	byg:wailing_garth	byg:warped_desert
cinderscapes:ashy_shoals	cinderscapes:blackstone_shales	gardens_of_the_dead:soulblight_forest
incendium:weeping_valley