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To turn vanilla mobs on or off, simply change that to true/false.
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The information here is vastly outdated, as Pixelmon has had 6 major version releases since then, across 9 major Minecraft versions.
To spawn Vanilla mobs in Pixelmon 9.x, you first re-activate the Vanilla spawning system by setting the "allow-vanilla-mobs" option in the "spawning.yml" config file to "true". By default, those will be still be replaced by our Spawn Replacment feature though, which exists to catch any Vanilla mobs that bypass the regular spawning system, like ones generated with structures. That can be freely configured via Data Pack, by overriding the files under "data/pixelmon/spawn-replacement/". You can find an example that completely deactivates any replacements here: viewtopic.php?f=175&t=31343