A hardcoded set of biomes for each mob seems like the most logical way to go. Later on we can add customization to this, either global, or even per-world.
Hm, or wait, it might get interesting. Are there per-biome dependencies in mob spawning rates? Then it might be better to have a table of per-biome mob frequency.
Hm, or wait, it might get interesting. Are there per-biome dependencies in mob spawning rates? Then it might be better to have a table of per-biome mob frequency.