Where Wild Pets Appear

pet_spawn/wild_pets.json decides which biomes wild pets appear in, who appears and how often. It is written the same way as the spawn lists in vanilla biomes:

{
  "biomes": ["#c:is_plains", "#c:is_savanna", "#c:is_desert", "#c:is_swamp", "#c:is_snowy_plains"],
  "spawners": [
    { "type": "chiikawa:usagi", "weight": 3, "minCount": 1, "maxCount": 1 },
    { "type": "chiikawa:chiikawa", "weight": 3, "minCount": 1, "maxCount": 1 }
  ]
}

biomes takes biome ids, or biome tags starting with #. By default it uses the common biome tags for plains, savannas, deserts, swamps and snowy plains, so biomes of the same kinds added by other mods count too.

All eight characters have a weight of 3. Pets share the same spawn slots as cows, sheep, pigs and chickens, so the higher the weight, the fewer vanilla animals there are; the defaults make pets roughly a third of the animals spawning in these biomes. Players can also turn it down with "Wild pet spawning" on the mod's settings screen, without touching the data pack. To stop a character from appearing naturally, just remove it from spawners. You can also write another file under your own namespace to have some characters appear in other biomes as well.

This file is read when the world starts, so after editing it you need to rejoin the world for the change to take effect; /reload won't do it.