oo3server/config/alexsmobs/mudskipper_spawns.json

{
  "biomes": [
    [
      {
        "type": "REGISTRY_NAME",
        "negate": false,
        "value": "minecraft:mangrove_swamp"
      }
    ],
    [
      {
        "type": "REGISTRY_NAME",
        "negate": false,
        "value": "terralith:cave/underground_jungle"
      }
    ]
  ]
}