As of Pixelmon 9.0.9 for Minecraft 1.16.5, the config option for allow-vanilla-mobs no longer fully re-adds vanilla mobs to the game, in favor of the new datapack system.
This datapack will solve this issue for you, so you can find all mobs as if it were vanilla, while also keeping in the Pokémon.
While this was made for 9.0.10 Pixelmon, it should in theory work for more recent versions that come out. Regardless, I will try to keep on top of this in case any major changes are made.
Note: In the current version of Pixelmon, you may still need to turn on allow-vanilla-mobs in the config, as this change isn't fully implemented yet. I am not 100% positive on this, but if it doesn't work without the option on, then try that.