If you travel through your portal into the Otherside and only see sculk mobs with no wild Pokemon in sight, check your world setup and underground travel habits to get creatures spawning properly.
Confirming Mods and Data Pack Status
1. Make sure both the Cobblemon mod and the Deeper and Darker mod are loaded in your current game session, as the spawn rules depend on both.
2. Open your save directory via Edit > Open World Folder from the singleplayer menu and verify the Otherside Cobblemon Spawns Data Pack is inside that world's datapacks folder rather than your resourcepacks folder.
3. Inside the world, run /datapack list if commands are enabled to ensure the pack appears under enabled packs, then run /reload to apply its spawn data.
Triggering New Spawns Underground
1. Leave the immediate chunks around your portal if you visited the dimension before enabling the data pack, and walk further out into the dark biomes.
2. Move far enough away from previously loaded caverns so older entities can cycle out and free up spawn space.
3. Keep walking across open underground terrain where wild Pokemon have clear ground space to generate.